For this achievement, you should open AK-47. It'll makes this achievement get much easier. Firstly find battery but do not take it until run out of ammo. Just stay next to it and shoot zombies. When you run out of ammo, search for ammo and after, stay close to a battery again but don't take it again, until you run out of ammo. Keep doing this tactic until you get 100 kills. Achievement will pop up.