First and foremost, this guide may not contain the biggest spoilers, but it sure contains A TON of spoilers. There will be guns and items experienced players may not have heard of and, obviously, spoilers about bosses and how they work. I won't be using spoiler tags, so if you don't want to know these things, DO NOT READ THIS.




For the inexperienced players who are fine with spoilers, the rest of this section is an explanation on what master rounds are. A master round is a great item earned when you defeat a boss without taking a hit. (Hitting armor counts) They're essential if you plan on making it far in the game, so getting knowledge and practice with this is quite important. Also, you need them in order to get shortcuts. This is the last "immensely magical item" that you need, and can be given to The Tailor at any time. Getting all five will give you the achievement "Lead God," which unlocks the (arguably) most powerful item in the game.

Aside from dodging and memorizing boss attacks, which are absolutely needed, there are ways to aid the process of avoiding bullets. First, you should obviously save money to stock up on blanks just before fighting the boss, but everyone knows that. :D
You may not know that explosives not only destroy your enemies, but also their bullets. A well-placed shot from an explosive weapon can punch a hole right through their attacks. Not using your bombs on groups and saving them instead can sometimes be the best option. The problem? Most explosive weapons have VERY high damage but VERY slow rate of fire/reloading. Invest not only in explosives, but also things that increase your rate of fire or number of projectiles launched. (Hip holster/ Scattershot, respectively)
When you're out of blanks and there are waaaay too many bullets coming, just make sure the bullets never come close! Drop a black hole or reload your fightsaber, then do everything you can to make sure you get through that wave. (Little Sparrow mentioned another favorite item, the potion of lead skin, which will give you time to catch your breath and finish the boss!) Hindering the bosses by placing a turret or freezing them can also make them much easier. The problem comes in obtaining them. Most guns or items with the ability to reflect or divert bullets are A or S. The seven-leaf clover helps tremendously, but also be sure to grab a bunch of keys in the first three levels (And don't waste them!) instead of buying that C gun you don't really need. Ideal characters for doing this method are the hunter and the pilot.
If all else fails, guon stones have your back. grab them if you see them in the shop, or any other fast moving pet partner that will block bullets.

It seems simple enough, but the fatal flaw of many gungeoneers is to try to let their skill (And luck) carry them by having an ammo-efficient, slow shooting, super accurate, low dps gun. Eventually, you're going to make a mistake and get hit. The best way to minimize this risk and make the most of your blanks? Kill that boss, and kill it fast.
Bosses are big enough that a terribly innacurate gun can usually land even the wildest shots. Items like scattershot or eyepatch are a much better buy than scope. While it seems like a good choice to get accurate items and guns, they're USELESS on bosses. Many try to stay far away from the bosses and snipe them, but that only works before floor 3. From that floor on, bosses will either be gigantic or will chase you down so you could hit them with every bullet of a sawed-off shotgun.
Guns that pack a huge punch for very little ammo are... not that good. Guns that run out of ammo in a single room, but clear it in the blink of an eye? That's what I'm talking about. Both? Amazing. Items like the hip holster can double your dps without using any extra ammo. Charge weapons proabably shouldn't be your pick, as they usually have a long time between shots... even if they use 10-20 shots to kill a boss.
Guns that ricochet multiple times often end up hitting a boss multiple times. Slow shots sometimes even damage a boss multiple times before going out the other end. Remember to value a defensive position over an offensive position, though. Be sure not to line yourself up for a good bounce shot if it means getting hit.
With bosses, items that reduce their health by a certain percent have a HUGE effect on how fast you can kill them. Items like the gundromeda strain that reduce their heatlh, in this case by 25%, can save you tons of time and ammo. Make this a top priority, as it also helps when facing the little enemies.
Oddly, accuracy can be better than damage in some circumstances. Wild or blindly fired shots can still hit home with the best guns and items. Backup gun shots can double a tracking gun's DPS, and items that allow 20% more bullets to hit are just as good as items that make those bullets do 20% more damage. However, don't try to keep your shots a perfect line. A good balance is what keeps your DPS high.

Not even the best players can always beat all the bosses without getting hit. Like, no one. If you say you can, the best supercomputers will be jealous. No, don't go to the comments right now... Stop! STOP! Oh well. Either way, even if you DO get hit, not all hope is lost. There are two magical items that will make you cry with relief.
NOTE: These items DO NOT stack. Getting gun soul will override clone. I recommend leaving gun soul alone if you have clone already.
This item will send you back to the start of the first floor upon death... Doesn't the game already do that? I wasn't finished... With all your items. At first glance, it seems like this doubles your chance of getting Lead God. In reality, it more than quadruples it. Say you defeated the Dragun and got hit on floor 3 and 5. Simply leave a room to kill yourself and you can replay all the levels with your godlike loadout. By the time you reached the Dragun again, you pretty much have anything you could ever want. Don't make the shopkeeper mad though, or he won't come back to the clone.
Unlock: Return Ox's arm to him. There's another guide here on how to do that.
If you feel comfortable with your starting weapons, this item will let you restart the floor you died on and fight through it to get your items back from where you died... an infinite number of times. With the notable exception of dying before you reached the place where you died. Then it's permanent.
Unlock: Simply beat the Dragun. How hard can it be?
Getting the fifth shortcut will allow you access to Boss Rush. You get three tries for every floor, except for the High Dragun. Saving the crazy OP weapons for the dragon and simply dominating all the lower, easy bosses will earn you Lead God fast. Also: You get a weapon for beating Boss Rush in the first place. And for unlocking it. It's worth it, people.

In case you don't know, save money to spend on blanks. (ESPECIALLY if you have full metal jacket) Be patient, practice, and slowly unlock all those powerful items that you're going to need for defeating bosses. If I confused you or mentioned an item you don't know, the wiki's there for you. Practice with dodge rolls and dodge roll upgrades. (It's a love/hate relationship with the bloodied scarf) but most of all,

PRACTICE

It's the single most hated and most effective strategy to do anything in this game.

Don't forget to like this if you like! Wouldn't you hate for someone to give up on this game because they couldn't find a guide like this? :P (jk it's not that good, but you can improve it by telling me things I forgot!)