To get the trophy that’s called “Overkill,” you must have two followers fight each other in the Fight Pits.
Make sure that you have at least two followers who aren’t doing any missions.
Next, command one follower to fight another by pressing
on his opponent for Pit Fight.
Go to the mission starting point, start the mission, then watch the fight go down. Once it’s complete, the trophy will unlock.
Have fun and good luck!