It's best to do this on Chapter Select and "Easy" mode. Choose Mission 1, Chapter 2 and save all your "gangsta fire" for the trophy. You'll eventually come to a cut scene where an enemy will blow up the exit. Run back in to the room and look up and you'll see 2 enemies standing on a balcony. Wait for one of these enemies to pull out a grenade and enter "gangsta mode". Shoot as soon as he throws the grenade to have it blow him up. This is very tough, so at least make sure you save your "gangsta mode" so you'll at least have a couple of tries.