This trophy requires a 100 hit combo, which can be pretty touch. To make it easier, start Chapter 8 and reach the part where you get to use Rachel and you find the small violet-colored fiends (the ones that glow around the shoulders). Using her projectile weapon, the machine gun, press and hold L2 to lock on and R2 to fire. Repeatedly tap R2 as the gun only fires in bursts and is not fully automatic. You can come close to losing the combo if you don't. Take out all of the enemies and you should get the trophy after scoring about 120 hits.