Walkthrough for the last level
Here's what to do on the bart bit of the level
Think of the umbrella as more of a club to smack the lightning with-even hitting it from the side will prevent it form striking Bart. Quickly sliding it from side to side pretty much does the trick.
Bart
For Bart's turns, position yourself so that the kite is between you and the flashing cloud then shoot it to push it towards the cloud. Ignore the aliens.