Kill Rockstedy Without taking a hit.
When you fight Rockstedy jump on top of the ledge on the right side of the screen, switch to Donatello and just attact downward. Everytime Rocksteady jumps he will take damage and you will get by without a scrape. This will work faster if you time your attacks with his jumps.
PC version
In PC version do this: A + S + D + F + G + H activates a walkthrough-walls code and pressing Q + W + E + R + T + Y + U + P activates an invincibility code.
Remove Crushers in Level 3
Go to the third level and hop in the Party Wagon. Whenever you see a pesky Crusher drive around, press SELECT. You will exit the car, and the Crusher should vanish from the screen.
Secrets
Easy way to restore power
Find a doorway that has a pizza slice or full pizza right at the beginning. Enter, grab the pizza, and exit. If you go back in the pizza will regenerate. Use this to restore power to all your turtles.
Share Boomerangs
When you get the boomerangs, throw up to 3 of them in the air, then switch turtles, and have the new turtle catch them. The new turtle will be able to use those boomerangs without having picked up a boomerang icon.