This is one of the trickiest achievements in the game. The best level to attempt this on is Home Level 11. You need to fire the shark at full power towards the closest group of three underwater ducks, aiming for the middle one. Your Shark will eat all three of those ducks. You also need to time it so the green soap block that automatically falls at the beginning of the level hits your Shark while underwater to push it to eat the other three ducks. This will take many attempts just to get the timing right. See the picture below for the best angle.