Solution taken from Clement_RKG on Steam:
https://steamcommunity.com/sharedfiles/filedetails/?id=32164...Start an endless game and survive until the slime appears. It is the 3rd boss of the game and should appear around 7 minutes into the game. The Slime will summon targets that follow you and leave behind some green trails that inflict damage if you stay in them. You will unlock this achievement by staying in and dying from his green mud.