You encounter the Corpser in Act 2. Chapter 8: Trapped.
The Corpser attacks include. Sweeping a large area with its legs. Spits out about 3 proximinity mines. As it gets closer to dying it will sweep 2 areas and spit even more mines. And it will use its legs to shield up. You can shoot. Or use explosives to open the legs.
There will be emergence holes that will spawn. If you can. Try and plant a grande prior to it opening up. This is An achievement as well if you have not gotten it yet. Drones and wretches will spawn periodically. Take care of them so you dont get overwhwlemed.
Make sure when you can to take advantage of executes/chainsaws/bayonette charges you get extra turn for your squad.