The information in this guide is what I find works for me, and is meant to help you formulate your own strategy against the giants, as my way is definitely not the most optimal way to deal with them.

First things first, when it comes to fighting any of the bosses, high damage sustained DPS is important. These things have a ridiculous amount of health, and quickly regenerating armor as you have probably noticed already when trying to fight them.

Mech composition is going to be pretty key in terms of dealing with them. Your going to need atleast a couple repair mechs to manage the damage. And for the 1st and 3rd boss, its very helpful to bring double C-rams on atleast 2 mechs. 3 if you want to almost completely nullify the missiles.

Powerful clustered shields are an absolute must with the CPU buff. And shield modules can be extremely helpful in defending against operarius, allowing you to focus more on the boss itself, however its optional if you have a very powerful chaff clearing mech in your team.

In terms of armaments. Ive barely touched the Size+ ones. So I will be basing my effectiveness off of the standard models.

Cannons and railguns are likely going to be your bread and butter for dealing high damage against the bosses through its armor. Their high armor penetration and damage is excellent for chipping away at the health constantly while its still got armor, and the longer the projectile passes through the body of the creature the more damage it will deal. For the cannon, Id pick between max damage shards and rocket assisted, with kinetic ROF to counteract the slow ROF to attain that high DPS. Shards will be excellent at stripping away armor and dealing high damage, but you will lose the ability to deal damage through the armor. Railguns on the other hand, can still deal damage through the armor with shards if you redline them enough. However, projectile acceleration is a pick you can never go wrong with. As it allows you to stay far away, and have more than enough pen to pierce through the bosses, which can be handy if somethings annoying you behind it.

On the other side of the spectrum, lasers, teslas and gatlings can deal an absolutely absurd amount of damage if built specifically for the bosses. However your going to have to deal with the bosses armor first before they become effective. Redlined lasers and teslas especially can melt a bosses health away with the laser power CPU. I can only imagine how powerful a size+ gun would be.

As for explosives. I havent had much luck with them outside of the heavy rocket and of course, the particle emitter, due to the small rocket pods chewing through ammo too quickly to keep sustained fire up for a long time. As for the mods for these. I stick with shards only with max damage and no fire rate. Especially on the particle gun, as it likes to pull enemies towards you which you dont want with the bosses. However they are excellent for stripping armor and dealing an absolutely bonkers amount of damage with the shards once the armors gone. And using shards is beneficial as it reduces the explosion radius, making it harder to friendly fire when the bosses get up in your face.

Thermal weapons however, dont even bother with them outside maybe the plasma launcher if your feeling risky. The short range makes it extremely dangerous for your mechs outside maybe the centipeda fight, as it can be good for dealing with the operarius. But its overall too dangerous to be used effectively.

Out of all the bosses, this bugger has the highest chance to outright destroy your mech if you allow it to get close. Alot of its attacks are able to be defended against, but make sure you do not let it get close and start biting your mechs. As it will only take a few hits to take down a fully armored castle.

I try to keep it at a distance at all times. Focusing fire on the legs to slow it down, then focus on its core once its unable to keep up. Its handy to have melee mechs here as the boss can grapple your squadmates with beefed up fragellum tentacles. And bringing C-Rams is extremely helpful, as the Araneus likes to pelt your squad with missiles often. However the boss seems less affected by the particle emitters gimmick of pulling creatures closer. So this is a great boss to blitz through with them.

Shards is very strong here against the legs.

Titanus Centipeda is a bit of a weird one to fight. It doesnt particularly have any powerful direct attacks outside of its flos like energy waves which can be very easily countered with strong shields. The main threat of titanus centipeda is its high movement and the ability to encircle your squad while spawning hundreds of operarius. Chaff clearing mechs excell at keeping you alive here so make sure you have one or 2, or a few mechs with shield modules. The fight will get WAY harder if your in an infested zone or in a nest as it will buff the operarius making them much tougher.

I recommend bringing lots of weapons with high velocity projectiles. The only part of the boss you can harm is the head, and with its very fast movement, it can be a real slog to kill if your projectiles cant reach it in time before it runs past you. Make sure you micromanage your mechs often to make them focus the head and not the other segments of the body. Many shield modules can make this alot easier as you can let the shield module damage mostly deal with the operarius while you focus down the head.

I do not recommend bringing particle emitters for this boss if you can help it though. As you only really get an opportunity to damage it thoroughly as the boss is charging towards you, and hitting it with the emitter will make the window of attack far far shorter as it pulls the boss closer, and potentially cause friendly fire. And make sure your constantly maneuvering to avoid being circled.

Titanus Monstrum can be either really easy or really deadly if you dont play your cards right. Its a static creature that has a giant version of the Colubra's shield. Kinetic weapons are your friend here til it brings its shield down. Especially highly penetrating ones. While its shield is up it will fire a blast of bullets at you. Dont worry particularly much about these unless you have poor armor. Focus fire on taking down the colubra that are in the vicinity, but try to stay in one area. I usually use railguns to fire through the monstrums shield to the colubra behind. Taking them down is very important before the monstrum drops its shield as the colubra are firing their energy balls from across the map.

Once its dropped its shield, it will do a large cone of light that will damage everything in it and overheat your mechs. This is the same as the moving light beam hazards on missions but stronger. It pays to have your mechs built with lots of cooling in mind for this or the fight will be a massive headache. Shields will defend you from it, but it will quickly whittle them down. The more stacked shields you have with the cpu buff, the longer you will have and it will save you from tons of damage. There isnt alot of cover to protect yourself from the beam so its best to prepare for it, unless you get one of the tentacles it spawns to make a protective wall for you. While it is beaming you, operarius meteors will spawn constantly. I recommend you constantly make your mechs focus fire on the monstrum, trying to damage it as much as possible. And liberally use your artillery strikes to keep your mechs clear of operarius. As the monstrum is most vulnerable now.

Energy weapons like the shards tesla will do incredible damage to the monstrum so it pays to get up relatively close. But watch out for friendly fire if you bring particle emitters. The boss is a huge target and your usual weapon settings to not fire when to close wont really proc here so your mechs may nuke themselves to oblivion. C-Rams are also nice to have as the monstrum will rapidly fire missiles out on occasion similarly to the Titanus Araneus.

This boss battle is a battle of attrition, you need to make sure you can put out as much damage as you can to end the fight quickly and avoid critically damaged mechs and pilot death. If you let it drag out and dont take care of the operarius and colubra you may suffer very heavy losses as your mechs try to defend themselves while being cooked alive.