To earn this achievement you must sever 500 limbs from the enemy Necromorphs or Human Soldiers.
The removeable limbs are :
Head
Arms (Left/Right)
Legs (Left/Right)
There are some great weapons for this including the Plasma Cutter, Machine Gun, Ripper etc. And, obviously, some don't work so well such as the shotgun and flamethrowers. You'll figure out what works for you pretty quickly.
I hit this achievement in Chapter14. Just aim for the kneecaps to sever legs on Necromorphs that move fast, or arms on soliders who can shoot you. Shouldn't take you too long to obtain.