This requires only the lancer.

Keep your distance from the Corpser and wait for it to lift one of its legs, when it does this shoot it in the stomach.

The Corpser will stop attacking and start to show pain, when it does this shoot it in the face and it will stumble backwards. Keep doing this until it cannot stumble back any more.

It will still try to attack you so stand on the smashable clamps indicated in the level, when he goes to attack you dive backwards and it will smash the clamps in 2-4 hits.

It will then fall into the water/acid/burning stuff and melt to death!