The following is a list of every Boss and the recommended strategy:
SEEDERS You should have the Hammer of Dawn, so just hit him with that. It should take about two hits, but watch out for the flying locusts they shoot out.
BERSERKER Once she appears (by running through a wall) immediately jet right into the next room, then go to the end of that room and there will be a closed door. You're going to have to angle it so she will charge and hit the door, so once you think you're ready, pop a pistol shot into her and she will start running at you. If you move away too soon, she will turn to try to follow you, so you have to wait till she is about 6 feet away then do a dodge roll (Left stick left or right + A button) Continue to do that until you are outside, then get her to run towards you and do the dodge roll again, then take out the hammer and hit her with that… Rinse and repeat until she is dead.
CORPSER I used the 6-shot burst rifle and the shotgun, so all you have to do is wait until he lifts up some of his legs (he will do it on either the left side or the right side) then go over to the lifted side and shoot him in his under belly (took 3-4 shots with the 6 burst gun) then he will like scream and raise his head up. You have to shoot him in the chin/neck and he will back up, follow him and keep doing what I said - after about two times of getting him to back up, wretches will come out, this is where I used the shotgun ( since the melee with the shotgun is a one hit kill to wretches). So do that, then go back to the corpser and shoot it in chest and then neck, so when he is at the very end of the giant hallway, he should be in a huge platform being held to the part that you're on by clamps. Get his smash attack and have one of his leg s hit the clamp. Once both clamps are destroyed the cut scene will come, and the Corpser is dead.
2nd BERSERKER(green house) All you have to do with his one is get her to hit apart of the wall – it will break and sunlight will come through. Get her to that spot (or around it) in order to use the hammer of dawn.
3rd BERSERKER(train) Get to the end of the train, there is a way to get the back part to open, - it’s either with a button or you have to get the berserker to run into the back, I don't remember which, but once you get the back off, just have her run into it and she will fall off.
RAAM SINGLE PLAYER: Get the sniper rifle and when the Kryll come off him head shot him, when he starts to get close run out and tag him with a grenade then got to the other side of the train and do again, you can use the machine gun if you run out of ammo, also at the opposite of your entering point there is an ammo crate. Do this till he’s dead.