This is how I got the 'Pacifist Pride' achievement.

Details: Clear any level 1-12 without using your gun.

This one isn't too bad if you are clever and have the power claw. Grenade upgrade doesn't hurt either.
This one is easiest in area 5.
You are allowed to use your grenades and your power arm to grab enemies and kill them. Make it to the top and kill the tank to win this one. Use the iron boots to swing into enemies too.