75 enemies killed with the rifle.
The rifle is the third weapon you get and is in the slot. You must get the most kills with this gun out of all of the kill based achievements. Upgrading it will help since you'll be using a lot. You should focus on getting these kills randomly when in gun fights when stealth is not an option and there are many enemies to dispose of.
Note: You cannot reload checkpoints to gain extra kills towards your stats. These must all be done on unique enemies.