On FSC ( Charred Court, in one of the rooms on the bottom right, as you see in the picture below ), on one of these rooms you must eliminate all enemies had a Slag Walker spawn outside of the area, therefore making anyone inside of it stuck.

If a player is going solo \ everyone is inside the room, this can pretty much softlock the level.

https://gyazo.com/d13d0c09197833e949e889a6f676f2e4