This is a power that you acquire later on in the game after completing a few story missions. You get basic training in the arena but the kills there also count. when you have this ability you'll need to press L1 while aiming at an enemy, this will throw them up in the air in slowmo. simply shoot the weak spot of 25 stasised enemies and you'll get this trophy. after unlocking the ability it should only take an hour (max) to obtain the trophy.
Happy gaming