Alternate between shooting the unarmored belly and the open mouth.
This will make the Corpser retreat till it reaches an area which is held on to the main floor by clamps.
Stand near the clamp and wait for it to attack you and destroy the clamp in the process.
Do this for both the clamps and you will trigger the cutscene for the Corpser's death.
Oh yeah, wretches drop from the ceiling twice - so, watch out from them and kill them before resuming your attach. And ignore Dom!