As the guide implies, in this guide i will go over some general boss tips. First off, as i (and darkvigor) said before, stay on the ground as much as you can. You can shoot from the ground, you can't shoot in the air unless you walk off platforms.

You may have noticed that most of us shoot at the boss as they spawn. A little-known fact is that the boss is actually invincible during their intro animation. Shooting at them is still recommended though, as their invincibility stops when their image appears, not when the boss starts. This means if you get lucky you can shoot and damage the boss in the background, getting in some extra damage before the fight. Beam weapons are even more effective at pulling this off, as you don't stop shooting the beam when the image appears, sneaking in extra damage. That said, if you have a charge weapon like the makeshift cannon, you're better off just keeping it until the image vanishes, as even if you shoot in the sweetspot and damage the boss, you'll spend the beginning charging it back and it ends up being ultimately pointless. If you play cultist (as most of us do) you might be able to land in some more hits by holding your mouse on them; if the game decides to change your gun, you will get some extra damage from tossing your previous one.

This isn't a general tip, but you should still know it: when the last dragun goes into the background (sniper rifle and RPG) they don't take damage. If you have a charging weapon, just charge that up instead and shoot when the dragun is back. If you have quad lazer, shoot up where the dragun normally is. Focus mostly on dodging regardless of what gun you have.

That's all i have, not much because the bosses are so different. I'll probably do a walkthrough for every boss sometime.