You might think defeating Ustanak 20 times in Predator would be a team thing meaning who ever defeats him...you and the rest of the team should get the defeat, Naw thats not how it works sadly..it would make this so much easier if that was the case.
Yesterday night me and my buddy played Predator so many time...and so many defeats...whether it was you defeating him or a team mate. After a while I starting thinking "maybe this has to be done by u personally". And YES I believe thats the answer. I came to that conclusion when we checked the records for Predator and I had 2 defeats on Ustanak.
This is my first ever solution to any achievements. It's pretty legit if you ask me.
P.S.- Boosting method would be to setup a game with one person. Then take turns killing eachother as Ustanak. Do it 20 times and the achievement is yours people. Happy achievement hunting to all.