You'll need to get the final blow on an enemy with a thrown item while they're laid out on the ground.
Personally, I found the ice blocks just after the Hillcrest Park area of the first level to be most helpful since they slide more easily than some of the other items and often bounce back off the edge of the screen and may give you a second hit for your throw.
When you're attacked by a pair of dogs, and then a few humans in front of a garage, there's a blue diamond looking object. Pick it up with
, attack the enemy of your choosing with it until you see them fall on the ground. While they're down throw the item with
. If they've taken enough damage you'll defeat them and earn the achievement. If this first block doesn't work, there's a second one shortly after (Either go through the secret door in the second garage or go straight through the snowball fight scene) where you'll get attacked by a larger group of enemies with an ice block waiting for you.
You can always do this at any other point in the game with any item of your choice, but for me the ice blocks made it a little more likely to work.