How to kill trapped zombies on Minecraft?

I built a simple experience farm, 3x3 hole below a spawner with water to push them down. They almost die at the bottom and I would like to build something to finish them all off with the flick of a switch. The ONLY problem is that pistons are not an option. I also do not want to use Lava or TNT. I just need to deal some extra damage. Any ideas?

Make zombies.

What you're asking is basically impossible, unless you were to build an automatic Dispenser turret with arrows, but it wouldn't be as instant as the flick of a switch.