Here is my 100% Achievement Guide for The Sinking City.
This can be obtained during the 3rd main mission of the game "Quid Pro Quo".
In Reed Heights, you will be tasked to go to the University. When you arrive, talk to the guy behind the reception desk who basically barters with you and says "Get rid of the monsters in the basement"..
Go down, and you have to get rid of a few rnaomd monsters, including a few stygians (Spider Like Creatures, Easy to kill) & Lethians (Things that spit vomit or something at you).
When they are down, next to a toilet door you see a red button.. Pressing that 3 times reanimates the blob like creature that was in the next room and now we have to kill it!
Personally, i used the machine gun, 15 shots of ammo is all it took, but you can throw a grenade or two at him, then finish him off with the shotgun or revolver.
He is strong, so just keep backing out and around into the rooms so he doesn't get at you, but he isn't too difficult a fight.
If you want to see how this is done, go to 2:03:50 in my Part 1 video.
Hope This Helps!