i got this yesterday, i managed to get it on the large bridge that goes out over the water but i must have kicked about 50 or so of the damn thing before i realised that they weren't counting. I kicked another one off and the zombie disappeared before it hit the ground and i had no power/agility points show on the screen. it seems too me that if the zombie falls to far it disappears so i coaxed a few back towards the beginning of the bridge using fire crackers and insults right around where there is a stair case up ton the north side of the bridge. It worked like a treat. every zombie i kicked over the side made contact with the ground and didn't disappear into thin air, also i noticed i was getting points on towards power/agility bars for the zombie dying from the impact. Also when i checked by snapping the achievements sure enough it was going up with every kill. Just make sure when you do it they fall far enough to die but not so far that they vanish and you don't get the points, as it says in the achievement description "Kill 100" not just kick them off somewhere. Make sure you have the drop kick bomb perk swell, this help kick the buggers over the railing in a most satisfying manner. good night, good luck.