This trophy is more annoying than anything else. For this, you have to fight a bunch of Shepards' in the Combat arena on Normal/Hardcore/Insanity.
You can do it the hard way, actually doing it on normal (hardcore or insanity), or drop the combat down to narrative and head into the simulator.
You have to have bought the Mirror Match Combat set from the gold winning terminal.
So choose that under enemies, choose a map, and some squad members and fight.
I chose to use the Wingman map because it's open and easy to maneuver in. I also chose to turn on the Player Damage Boost score modifier to make the match a bit easier.
Squad members chosen were Liara and Garrus. Liara for her biotics and Garrus because I just like Garrus. As I was an Engineer, I threw out a lot of Combat Drones and Sentry Turrets. As for weapons, M-7 Lancer is great because of unlimited ammo.
The simulator Shepards' can be a bit annoying because they like to spam their attacks and run up to you. Try to avoid them and find good cover; their biotics and powers are pretty damaging as they cause staggering and can make you vunerable, even on narrative.
Go through the waves and once you hit the last one or two enemies, bring the combat difficulty up to normal and finish the simulator. Trophy should pop as you load into the arena lobby.