Huh. You know, just in case anyone forgets to look at the combo card list, there is one in there that isn't being mentioned in this thread or in GuerrillaTechs list. If you combine a floor buffer (some right by maintenance rooms) with a stun baton from the zombie wranglers, that was my final combo and I popped the achievement with no problem.
I didn't use any of the crazy methods of deleting saves or only making the new weapons before a certain case. I made football grenades and the cement saw plate thrower right in the middle of all this and finished this achievement in the middle cases of the game. Hope this helped.