Lure the zombies to a puddle of water and use the trap. Consider doing this at Old Town. Look for large puddles of water with a small generator nearby. Usually they are at the large town squares. Stand above on the big lamps and throw some firecrackers to lure them into the puddle. Then, jump down and activate the generator. Unlock the electric fence trap upgrade. Stand above an electric fence trap, throw some firecrackers down, then activate the trap.