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.

His best weapon, so it is easily gotten when you go for the 4 achievements where you have to beat the mission with both War Machine and Iron Man. The shotgun one is a little harder and seems to be better to do after you level it up.