Trying to play through the zombie mode. Just started today so my guns aren't the best. Anyway skinny..

Trying to play through the zombie mode. Just started today so my guns aren't the best. Anyway skinny 1,2,and 3 were easy. Fat 1 and 2 were easy fat 3 I cant kill one of them? Supposed to kill 18 or something and I've gotten them limbless but they don't die. Unloaded dual pistols, a shotgun, and 3 of the 2nd level grenades, and nothing? Do I just need a better pistol or machine gun maybe? I freaking hate wasting energy especially when every level you try and play has a stupid ad attached to it