Iron Man 2

A good area to grind kills for weapon achievements is Level 3 at the start. When going through the level down pass the lasers trigger them off to make some enemies spawm. Continue on and you will come to a door with 8 machine gun turrets shooting at you (Four on each side). Shoot the turrets andyou will get a checkpoint. Quit the level and load up again and repeat to get the achievements. If you quit before the checkpoint the kills won't count towards the achievement.