Blast 10 zombies at once with a Cherry Bomb!
I found that the easiest way to do this was to have Cherry Bombs attached and wait till a huge wave of zombies appeared. Then place this is the middle of a large group, Hopefully ten zombies will be in this. If not just try again. Shouldnt take too long to do.
If you havent managed to get this in your playthrough of Adventure mode you can do it in versus mode. Make sure you have enough cash for a cherry bomb, and the person who is controlling the zombies has enough to call in 10 zombies. Then call in 10 zombies an plant the cherry in the middle.