Discovered by: (before 10/2016)
Tutorial videos: Avouca
Cruella has a blind spot, and you can take advantage of that to manipulate her movement.
Shoot the wall (1/6) and go all the way to the left once you're able to move. Reload the catapult and shoot the wall (2/6).
Go to the right and stop about halfway to the right end. Cruella should be going towards the puppy from the right side. Stand still until she notices the puppy. Start moving towards the right end as soon as she throws the water balloon.
Reload the catapult as fast as possible and follow the exact same movements as on the video. You'll know that the trick worked when 3/6 is on screen AND Cruella goes all the way to the left. If she throws the water balloon at this point, then the trick didn't work.