Iron Snout

During any game mode you're likely to see plenty of enemies riding rockets across the top of the screen. For this achievement, you'll need to jump up under a rocket enemy and hit him from below, which will send the rocket crashing to the ground. Strangely, the rocket explosion has no splash damage, so it's going to need to connect directly with an enemy. This will happen many times throughout your grind to 1000 kills though, so no need to worry about timing it right. Just make sure you don't jump up too early or the rocket will explode on you. Keep kicking down rocket enemies and one will definitely crash into an enemy.