Here is my guide for the bosses in the Rise of the Triad remake. I played on normal difficulty with Taradino. The strategy might change on higher diffuculty levels, but not with a different character.

In general, the bosses are not too hard, but just annoying since they can kill you in a few hits or instantly. A few general tips before we get started with the specific strategies:

• Use the Bazookas: Every boss level has them and the other weapons don't do enough damage to take the boss down in a reasonable amount of time (with exceptions noted later).
  
• Keep moving: Requiered skill for Old-School FPS. A moving target is harder to shoot than a stationary one. Take as less hits as possible as health is in short supply.
  
• Learn the patterns: There is no special creativity requiered to beat the bosses, just learn their patterns and the fastest way to kill them.
  
• Expect to die: I don't think anybody can beat the bosses on the first try, but here, you will most likely die multiple times trying to beat each boss. If you're a really good player (or playing on a low difficulty), you will probably only die a dozen times or less. I stopped counting, but died a lot.
  
• Be patient: Patience is key to beat the bosses. Since they can kill you very fast, just rushing them will only get you killed. Wait and observe what the boss does (patterns!) and react accordingly. It will take some time (even with his guide), since the bosses are hard by default.
  
• Keep cool: When dying often in a row, you will notice that your skill drops because you get angry. Your accuracy will lower and you will die even faster since you make mistakes like shooting the wall. Now choose your strategy: keep playing untill to got the pattern down or stop, do something else and start over with a fresh mind. I prefer the latter, because I tend to smash my keyboard and mouse when I'm angry. But do whatever works for you.
  
• Practise, practise, practise: The mantra of FPS-Legend Johnathan „Fatal1ty“ Wendel is more important than ever: Even if you die, try to learn from your mistakes. If you get further the next time your are on the right track. A bit of a downside is that you can't start at the phase of the bossfight where you died, but have to start the whole fight again. Try also to improve in the early phases, since when you survive them bearly you will probably die at the very beginning of the next.

Experienced players of Old-Shool FPS probably don't need this guide or want to figure the bosses out them selves, but I hope it helps newer players without the experience of games like Castlevania, Ghosts'n'Goblins and/or Ninja Gaiden on NES or classic FPS games like Doom 1+2, Quake 1+2, or Duke Nukem 3D. The game is certainly harder than your average modern FPS, but not impossible to beat and definatly not Nintendo-Hard. Normal levels shouldn't be a problem, but since the bosses gave me some headache, I decided to write a guide to hopefully save others the trouble.

The guide is based on Version 1.0, mentioned bugs and gliches can be patched out by now. Just check the patch notes in case, especially when the last updated date was some time ago.
Feel free to comment, make suggestions, I might add them after I have tried them myself. Also feel free to point out mistakes, just keep the flames out, we are all mature here, right?

Before entering the boss area, you can grab an Bulletproof West in one of the side rooms on the way aswell as an Asbestos Armor. Be sure to grab both, you will need them in the early stages of the boss fight. Also, you can get Flamewall-Gun near two rotating saw blades, grab it without taking damage - it is possible, but requieres some practise and maybe some luck. If you take damage, just reload the map, it's not worth it.

The boss room is of rectangular shape with three doors both the left and right sides (from the point where you enter). Ignore the last one on each sides, they are both dead ends without anything helpfull. The others lead to a room with six Bazookas (three on each door), six Priest Porridge and a Heat seeker in one room and a Split Missle in the other. Note that there are flame-traps at the door, which will activate very frequently.
I tend to ignore the weapons because of one simple problem: the automatic change on pickup (why the developers haven't implemented a option to deactivate that is beyond my understanding). The Bazookas are right at the entrance, so you will pick them up accidentially if you hug the wall (also wasting ammo). If you got another weapon, it will just change over and over, because it gets stuck in the doorway. Now, it doesn't always change, but according to my experience, It changes always when you don't want it to and not when you want to. So, I saved myself the trouble and just ignored them. Afterall, they don't do that much extra damage (the Heat Seeker even less) to really make a difference.
I suggest to pick one rome for the first phase of the fight, doesn't matter which. Switch for the second phase.

After entering his room the fight starts with a short phase, which I call Phase 0. The doors will open and bunch of mixed Triad-Soldiers will come out. Just take them out with the Flamewall, just switch sides between shots. You can empty it, since you don't need it later, and you will probably have to. If there are any soldiers left, kill them with the MP4. Then, Phase 1 starts. Head straight to one of the rooms on the side and grab a Bazooka and stock up on health if needed.

General Darian is up at the ceiling in some kind of flying booth. In this phase, every ~20 seconds a wave of electric energy will pass through the room back to front. It does moderatly damage, but can be avoided by simply jumping over it. If you look the other direction, listen for the electric buzzing-sound. With some practice, you should be able to jump over it just by listening and without taking any damage.
Also, but way less often, he activates some moving fireball-throwers at the sides of the room. Shortly before this happens, you will here an alarming sound. You can try to dodge them all, but I prefer to run for one of the rooms where you are safe.
Darian does nothing else, just shoot at him until his healthmeter runs out, dodging the hazards. I haven't counted them, but I think it takes around 8-12 hits to take him out. But the fight is far from over.

Now we are getting serious. After his health is drained, head straight to one of the rooms, preferably the one with more health, because you going to need it.
Darian stays in his booth, health replenished, but now shoots barrages of missiles. Two direct hits kill you, so do 3-5 splashhits. So keep in cover when he starts shooting. Don't go out in the open, because he will at least hit you and you will take a lot of damage, also, more stages-hazards:
The electic wave is gone, but instead the flameball-throwers are activated much more frequently. Also, at the same time heated spikes rise from the floor, killing you instantly.
With all those, I prefer to stay in one of the rooms and just peak out when the boss is not shooting, getting one or maybe two shots at him. A problem are the flametraps at the doors, so I wait until he stopped shooting and the flames are just out. It requieres no skill or strategy, just patience.
You can anticipate where he is by watching his rockets hitting the wall. He will shoot directly at you no matter where you are (like he was a wallhack), so watch the little parts of the wall (which are by the mercy of the developers indestructable) flying around on impacts. But beware if he is across the room and aims straight into the doorway, he will also hit you if you are not carful. The safest place is right in front of the TV, but you can't see anything from there, so keep moving in the room to get an idea where he is. When you shoot, you only have a few seconds to move, aim and shoot, so better be sure where he is.
Like Phase 1, it takes like 8-12 shots to take him down. Litterally.

With his cabin destroyed, Darian will be on the ground in the middle of the room. Grab a Bazooka and all the health you need and get out. Just follow three simple rules:

• Don't stop moving
  
• Don't stop moving
  
• DON'T ♥♥♥♥♥♥♥ STOP MOVING

In this phase, he shoots tiny missiles, but they will kill you instantly now matter how much health, armor and/or goodluck-charms you have. But they are easily to dodge, just run around him in a circle and never stop. Fire with the Bazooka and after it ran out, switch to the MP4 and keep firing while moving. Don't go into on of the rooms, or he will kill you when you head out. Or he gets stuck at a doorway, then you might live, but don't bother with it. Just keep moving and shooting, thats all.

The second boss of the game i think is a bit easier than the first one, since he can't kill you instantly. But it can only take him a few seconds, so he is nearly as dangerous.

You start the level in a room with a blood river heading a cylindrical room. This is the stage of the final phase, but were are not there yet. There is an Excalibat behind some bars, to open the door you have to use a switch located at the middle pillar in the blood river (Thanks Anthem of Grimm for the Tip!):
There are stairs to the boos room where you can grab some health and a Heat Seeker, take it if you like the weapon, but it is not mandatory.
The room itself is a huge cylinder with Doyle flying around in his chair-sorta-thing. There are three platforms at the side, two lower and one upper. You can reach them by using the bounce-pads, the lower ones easily while the upper reqiueres a bit more skil. My suggestion: go to one of the lower, then head left or right and jump on the nearby bounce-pad and steer your way to the upper platform. If you got through the challanges in E2L2, this shouldn't be a problem.
Every platform has the same structure: on one side, there are three Bazookas. In the middle, there is a nozzle where Doyle replenishes his energy every once in a while (you don't see a meter or anything, he just does it from time to time in Phase 1). To the left and right of it respectively, there are doors which are closed initially.
Initially, the boss will ignore you, just fire a few shots at him (MP4 is sufficient) and the battle starts.

Doyle sits in this chair protected by his indestrucible shield. Don't bother shooting after you get his attention, it's just a waste of ammo. He shoots tiny missiles that do a lot of damage, but can be dodged. You can see an orange beam where he aims, just keep moving to avoid it. After you have reached a platform, just move from side to side in front of the nozzle until his beam stops moving. Then he will shoot and destroy the nozzle, but only if the aims at it, that's why you shouldn't use the complete range of the platform to move. The doors on this platform will open and two Triad soldiers each will come out (or just stay in it, AI is not the smartest as you probably already figured). Just kill them to find a Priest Porridge and two Monk Meals inside. Repeat the steps on every platform to destroy his shield and phase two starts.

With his shield destoryed Doyle is now vulnerable. But he also fires with a chaingun, doing massive damage. Not killing you instantly, but still it only takes him a few seconds. But he has a pattern: keep in cover while he shoots, he will stop after a couple of seconds. Then get out and try to hit a Bazooka-shot. He will move around the room to another position, wait until he is stationary. Again, you have a few fractions of a second to fire the Bazooka at him. But get in cover immediately. Use the health in the tiny rooms if need, those are also good cover. Repeat the steps, as he will (like General Darian) take around 8 to 12 hits before he gets down. Prepare for the final phase of this fight.

Doyle will crash to the floor, destroying it and uncovering the blood-cover floor you've seen earlier with few pieces of cover. He is stuck at the middle of the room, but can still fire his chaingun.
You will get sucked down from the platform, but don't stand to close to the walls left or right, or you will get stuck at the tiny extra-walls and have to reload and therefore start all over again. Notice the half-damaged piece in the middle of the attached screenshot, thats the spot you want to avoid at the end of Phase 2.
There is not much you can do now, try to grab a Bazooka and health while he falls down and jump afterwards (or wait to get sucked down). Fire at him non-stop, just take brief cover behind the boxes, but don't get too comfy behind them, as they don't last long when he shoots at them. He will still fire and stop like in the phase before, try to use the cover when he's firing. There are two Bazookas at a door, grab them if needed and otherwise keep moving and shooting. If your lucky, you will kill him before he kills you. Don't bother with any other weapon, as they do way too few damage to kill him in a reasonable amount of time. That means, before he kills you. I got out with 5% health left after 6 tries.

After entering the level, you have a rather long walk before you actually reach the boss. Pick up the Firebomb weapon along the way, since it does the most damage of all explosive weapons in the game. It is most usefull against groups of enemy, but since the boss is a big robot, it will get hit anyway.

The area of the boss is a rectangular room with four pillars, each with a bounce pad pointing to the outside of the room. With those, you can reach the balconies where explosive weapons and health is located. Also, there are containers with health in it and bazookas at the side. More bazookas are located at all four walls. There are also small balconies at the pillars, but there is nothing helpfull. Note that some tiles on the floor look different (those plate-shaped things you probably seen earlier as platforms, bridges, etc). Those lower during the fight, the rest does not. The attached screenshot shows it from the outside, viewable before you enter the room through a lift (and creating the checkpoint)

This boss is a complete joke or the developers just got lazy. There is absolutly 0 (as in: zero) strategy requiered to defeat the boss. Just keep moving and shooting at him. Initially the boss is covered in an indestructable shield, it vanishes as you get really close to him and the fight begins. The boss follows you through the room, so keep moving and try to keep a good distance. It can't get through narrow space, but it can still hit you with its abilities.
The fight has no phases like the other, instead the robot as multiple attacks or modes:

The first one is easily recognizable, as a lot of blue beams leave the upper body of the boss heading to the ground. Don't get near them or they will kill you instantly.

In the second mode the boss fires target-seeking rockets, one at a time in ~ second intervals. Just run in a circle and try to keep a good distance to dodge them. Two hits kill you, get to cover if needed. But don't get to comfy, if the robot comes near you, the rockets hit you around the corners. Note that the health in the containers also gets hit and gets heated, granting twice the health. I don't recommend getting up to the balconies, as there is not enough room to move and you get hit easily.

The third mode is activated as the robot reaches the end of its healthmeter, around 25%. It will shoot rockets in multiple directions. Here you basically have to kill ist as fast as possible. I recommend using the Firebomb now.

Sometimes in between part of the floor will lower into the lava. If you have 100% health it will bring you down to 10%. If it happens you can reload, because as soon as you get up you will end with a rocket in the face.

With this boss and his high amount of health, you can see how much damage your weapons do. Don't bother with anything not-explosive, as it will take forever to kill him and he most likely will kill you first. A Quick overview:

• Bazookas do ~2% damage. Not very effective, but they lay around everywhere, so not too bad of a choice
  
• Heat Seeker do even less damage than bazookas, ignore them completly. If you really need the seeking-ability to hit the boss, you probably didn't get there.
  
• Drunk Missle: I heat this weapon. It takes some time before it fires in primary fire mode. It does a good amount of damage, but it takes a couple of seconds before it does and there is no indication that its charging up (I just call it that) so I always think it doesn't do anything
  
• Firebomb: does 8-10% damage to NME. This is what you want in the last phase, to kill it quickly.
  
• Split Missle: good damage, a bit more than the Bazookas. Only avaiable on the balconies.

I recommend to walk in and exchange your Firebomb for a Bazooka. Than run up to the boss and fire away. Keep moving and try to not to get hit. Learn the layout so that you can run backwards without getting stuck. If you get stuck in a corner, you almost dead. Remember where you feft the Firebomb and when the boss gets down to ~30% health, head over there, grab it and fire. If you get lucky, you don't get hit by too much of the rockets.

The final boss of the game has two complete separate forms. Surprisingly, I think he isn't the hardest boss in the game, as he can only kill you instantly with his meele attack, which is easily avoidable. But he has a ton of hitpoints, so it takes some time to beat him.

You start at a bridge surrounded by lava. As soon as you walk halfway past it, two Darkfire Monks appear on platforms left and right. By now you should pretty good at killing them, try not to take any damage from them. Head on to the area of the first phase.

The area has a rectangular shape, but the floor is filled with lava. You can walk all the way around it, there are bridges left and right and a ledge at the opposite side. There are multiple platforms floating around in the middle area. You can reach them by using a bounce pad or the Mercury Powerup, you see the first one right when walking in. Underneath is an Asbestos Armor, save it for later, you don't need it right now. On all of the floating platforms in the middle is one explosive weapon located. Two platforms are outside the bridges and reachable by just jumping on them. They have health in form of Monk Crystals and more Mercury Powerups.
When walking in there are round rooms on the left and righ side each. Both have a Large Monk Crystal and weapons, three Heat Seekers in the left and three Bazookas in the right one. They can be accessed by small hallways from the initial bridge.

El Oscuro appears in the middle of the room and flys around the room like Doyle.
He has two attacks: First he throws one big purple magic ball which does a lot of damage. Then he shoots a lot of smaller purple magic balls. Between them he flys around to a different spot and fires again - I bet the developers used the same code as for Doyle.
My strategy is to stay in one of the rooms to the side, because they provide good cover, similar to the fight with General Darian. From then on its pretty much the same as with Doyle: wait until he stops firing and shoot a rocket. Wait until he moves, fire again and get in cover. Repeat until his healthmeter runs out. This will take a lot longer than with the other bosses, a Bazooka shot only takes 3-4% of his life, so expect to use all ammo in the area and maybe switch rooms.
The Mercury Powerup can been an option, but it usually runs out at the worst possible time - when you are above the lava.

El Oscuro is not yet defeated. He flys back to where he was initially and opens a passage way. But before you jump down, stack up on health and don't foget the Asbestos Armor underneath the first Mercury Powerup, you'll need it in the second phase. The jump down, on your way you'll find a Large Monk Crystal, a MP4 and a Heat Seeker. Take it if you like and head on to the area of the final fight.

The final fight takes place on sort of an island surrounded by lava. To the left and right are penisulas with explosive weapons, Monk Crystals and eggs for Spawns of Oscuro. They will pop out as soon as you come near them or El Oscuro opens them. They are weak, but small and can deal quiet some damage in large groups. Try to kill them with the MP4, only use something bigger when the group gets too large.
In the middle is a plateau, which can by reached a stairway to the left. This is were El Oscuro is initially, behind it is a Dark Staff, one of the stronges weapon of the game.

El Oscuro starts at the plateau on the middle and marches right towards you. Move to the island firing at him, don't stand still. Like every other battle, it is important to always keep moving and don't get stuck, especially the penisulas are dangerous with all the stuff on it.

El Oscuro has four abilites:

• He can attack you with his claws, almost killing you instantly. Since its a meele attack, just keep away from him.
  
• He spins and creates a whirl of fire, dealing fire damage which the Asbestos Armor should withstand. It has a very short range and does moderatly damage, just keep away from him.
  
• He can pull you towards a random location. Sometimes when you are behind an obstacle, you can get stuck behind it.
  
• In the later stages of the fight, he throws a bunch of Spawn of Oscuro towards your direction. Just keep moving to avoid them, but you can survive if you get hit.

In his new form, he is only vulnerable at the sack-thing at his bottom. The hitbox for it seems very small, so the Heat Seeker or the Dark Staff, which lock to a target are very effective. But sometimes they instead hit one of the Spawns nearby, not much you can do about it besides killing them first with the MP4.
My general strategy was to stay up at the plateau, because he can't reach you there with two of his attacks and if you get lucky, you get stuck behind the bones coming out of the floor when he tries to pull you through the room. Most of the time he even just stands in front of it doing nothing, grating you free shots. It still takes a lot of shots, some equal amount as in Phase 1, but you have to hit the sack-thing which makes it harder. Fortunatly the Dark Staff does a lot of damage, you can deal damage around 25-33% of his healthmeter before it runs out of ammo. And since it locks in the targed its way easier than with the explosive weapons.
Otherwise, just keep moving and fire at him whenever you can. Surprisingly, the MP4 is not as ineffctiv as with the other bosses, using it when you run out of explosive weapons is not a bad idea. Not much in terms of strategy, but he was ton of hitpoints. A little pit of practice and knowledge about the area should do.

When finished him, einjoy the epilogue and the credits (altough they run for over 20 mins in total, but you get a achievement for sitting through all of it).

2013/08/14: added how to get the Excalibat before facing Doyle
2013/08/04: El Oscura section added
2013/08/03: NME section added
2013/08/02: Initial Version, first two bosses (General Darian and Sebastian "Doyle" Krist) covered