If you're having trouble getting any of the "X kills per weapon," or Headshot or Survivor trophies (kill 75 enemies before dying), there is an area shortly after starting "Chapter 19: The Siege" where you can camp and shoot repeatedly respawning enemies. After starting Chapter 19, make your way down the sloping alley to the big courtyard. There is a mounted machine gun uphill on the left constantly shooting at you. Clear out as many enemies as you can and climb up to the right of the door of the building closest to the cliff (on your right as you enter the courtyard). Take cover looking toward the mounted machine gun on the left and use a grenade to get rid of it. If you now look slightly right of the machine gun, at the small grass/mud patch before the next building, you will see an enemy running from behind a building from right to left head toward the machine gun. Camp here in cover and keep shooting the enemies as they appear from behind the building. They only stop spawning temporarily if one of them reaches the mounted machine-gun (if one reaches the gun, then grenade them to start the respawns again). Beware: it will stop permanently if you go further to the right, triggering the RPG/wall collapsing cutscene. If you've already completed Chapter 19 at the difficulty you're playing, purchased weapons in the store, and unlocked the "Infinite Ammo" tweak, then you can use these to get the weapon/kills trophies.