Don't do the Brawler trophy on Insanity. It's not worth the hassle. Wait until the Insanity run is completed and pick it up on a second, Normal playthrough. It really doesn't take long to get. Just punch then immediately shoot all enemies with a machine pistol. It drops in no time. If you are looking for a good place to get it rather than trying in every combat (which can be frustrating), there are two primary places.
ME2 SPOILER ALERT!
Site 1: Neith - Eagle Nebula/Amun
A crash site where Mechs have gone crazy. You have 128 LOKI Mechs here to get the trophy from. Send your squad away using the D-pad, hide behind a crate, then wait for them to walk up to you. As they arrive, run out, punch, then shoot with a fast firing weapon into their chest. That done, return to cover. You'll get the trophy in about 10 minutes at most once you have a hang of it.
Site 2: Capek - Titan Nebula/Haskins
This is the factory where all the crazy Mechs on Neith came from. It unlocks once you've visited the N7: Abandoned Research Station. In the lower area where all the crates form a maze you have infinite spawning Mechs. Get brawling. Once done, move through the maze of crates and the Mechs stop spawning.
