On level 16 you will have a level with a ketchup bottle dripping from the top. There is also a layer of ketchup in the burger at the bottom. You cannot personally destroy any of it. The AI will take chunks out, don't worry about that. The best tactic is to fly around without shooting anything, and the big enemy will take them out. Once you are down to 3 enemies, get they will spawn one at a time. Try to get them up against a wall. The second guy is the trickiest as he has homing missiles. This will take a few tries as you need them to spawn between you and the wall to avoid taking out any ketchup.