Clean Jersey Achievement
Don’t take any damage from Kaos' rockets during the mini-game on the Dread-Yacht
After completing Chapter 7, you'll be able to go downstairs to the ship's turret on the Gun Deck. When you jump into it, you'll have the chance to play of one of three randomly selected mini-games (shooting sheep, barrels, or artillery shells). For this achievement, you want the mini-game where you shoot artillery shells, so keep playing and completing the mini-games until it appears.
Once the artillery shells mini-game appears for you, carefully shoot down all thirty rockets without taking any damage. If you do get hit, allow yourself to fail, so you can replay it immediately. (Leaving or winning the area randomizes the game again.) This can be tricky, as the shells move fairly quickly. If you're having trouble, have a friend help you in co-op play, and have each of you take half the screen.
Here's a video by 2-4 Gamerz: