What you need to do is use the cactus's 3rd ability which unlocks at cactus level 3 and it is a little flying onion drone, and you have to use it's secondary/special attack (the Y button activates it) to kill 10 player zombies.
So this can be done quite easily in garden and graveyards, the reason this is, is because most of the enemy are always in a group together, either sniping or being muppets so just launch your drone and wait until you can use your special attack, once you can you want to fly around the map (but keeping high and out of sight because you only have 5 health) looking for a group of zombies (they have to be players) and then unleash the rain of corn mwhaha, doing this I did get a few triple kills and some double kills so you should get it fairly easily.
Hope this helps.