Dismember 1000 Limbs

Shoot for the head, arms or legs of aliens, after their dead run to their body and stomp off every limb until you just have the body left. Head, arms, and legs can come off. You can do this on normal dead bodies too. Rinse and repeat, it's pretty amusing.

You get Marksman Achievement by dismembering 20 limbs, and Surgeon Achievement for 500.