Very simple achievement. Though like the other 'Defeat ________' achievements, you can do this on Insane (the difficulty above it), it doesn't have to be on Hardcore. So if you plan on doing an Insane playthrough, you will snatch this one on the way.
So you fight the Corpser in Act 3 - Chapter 5: Angry Titan. When you do confront it, it will be near a ledge, and you have to keep pushing it back until it falls off.
When it has it's claws on the ground protecting it's face, shoot at it's belly, through the cracks between his claws. This is will hurt him and make him raise his claws. At this point, shoot his face and he will back up. Keep doing this until he is at the very edge, then a 'point of interest' thing will come up, and it shows where two smashable clamps are. Shoot these, and they will disconnect him and make him fall off.
It should also be noted that he does very little to attack you. If you get too close, he can slash at you, but if you stay your distance, you're good. However, wretches can appear at this part (though I think if you are fast enough they don't appear, I could have sworn when I did a co-op walkthrough with my friend, the wretches never came).
**NOT MY VIDEO**
YouTube credit to mdegast.