There lot of zombie and brick battle game. There badge for brick battle but not for zombie? How about a badge for zombie killing? Like, zombie killer: 50 kills ) Zombie kinda easy to kill ) zombie destroyer ( 200 ), zombie bloxxer : P.( 350 ).
the only problems with your idea is that zombies are user created and there is no scoring system for killing them plus people would cheat and make zombies in there place that cant walk and shoot them for kills