This cheevo is awarded for possessing 50 monsters during versus matchmaking.
To possess a monster you must be on the attacking side so therefore you must NOT be the host.
When the host starts the match hover over one of the monsters and press
and this counts as 1.
Now as soon as you have possessed the monster press
to jump out and then possess the next monster. Rinse and repeat.
This is accumulative and will come after a few matches.
Can be done in legit matches but far simpler to do of boosting with a partner when working towards 20 wins.