In Act 3 (Belly Of The Beast), you and Dom must fight a Corpser with attacking Wretches. Note: This is easier in Co-op mode. After the intermission sequence, have yourself or the other player kill the attacking Wretches while the other person shoots at the Corpser on the body part closer to the ground. After the Corpser raises its legs in the air, aim for the neck and shoot it. After doing that about three times, he will back up on a platform. Repeat the process, but do not shoot him in the neck. Get near one of the platform's latch. The Corpser will try to hit you but hit the latch instead. Wait for one side to break, then do the same for the other. An intermission sequence will start. In single player mode, do the same thing but you will also have to fight the Corpser and the Wretches a little more.
Walk across until the Corpser jumps down from the ceiling and you have to battle him. The quickest way to defeat it under any difficulty is to wait until he opens up his legs. When you see a gap, Roadie Run to his abdomen. A shotgun is recommended. Shoot him in the stomach until he arches back in pain, then shoot him in the neck. Repeat this until he flings you back as he moves away. Repeat this cycle until the "look here" icon appears, then shoot the couplings right when the Corpser is on top. Do not worry about the Wretches; allow your partner to take care of them.
A sniper rifle and Lancer combination is the best to use. As soon as it lands, it will not move. Once it starts moving its legs slightly, shoot its belly with the sniper rifle to make it expose its face. Using the Lancer, spray at its chin. Repeat this process until it is on the plank. Once on the plank, either you or your teammate can simply shotgun the latches and they will open up. One shot should do it. If you do not have a teammate, shoot the latch yourself with the sniper rifle. This is slightly easier with a teammate. Either way, this strategy will allow you to kill the Corpser before the wretches come out, and give you less things to worry about.