During the final battle against Shredder, Shredder splits up into two forms. They're identical in appearance, but only one is real; the other is fake. However, both forms are able to use the lightning attack that instantly kills you by turning you into a normal turtle.
However, there is a way to make the fight a lot easier. When either form is weakened to near death, their helmet flies off. As you're beating up on the two forms of Shredder, one of them will lose their helmet very quickly. When this happens, LEAVE HIM ALONE. This Shredder is the fake, and he cannot use the lightning attack without his helmet on. Only the real Shredder will be able to use the attack from here on in, but since it's only him using it, you can avoid it and slowly beat him down with ease.
For the rest of the fight, ignore the fake. If you kill him, the real Shredder will release another fake and you'll have to worry about lightning from both forms all over again.
Extra lives and stage select
At the Title Screen press B, A, B, A, Up, Down, B, A, Left, Right, B, A, Start.
Extra Lives without Stage Select
On the Title Screen, press Up, Right, Right, Down, Down, Down, Left, Left, Left, Left, B, A, Start.
How to defeat the annoying robots
You know when you are fighting those robots that shoot lasers and move really fast? Here is a way to defeat them easily. Wait near the right of the screen and when the robot moves to the right, try to attack it making it go right and making it bounce until it is defeated.
Stage Select
On the title screen press down on the control pad 5 times, then press right on the control pad 7 times, then A, B, Start.
Easier battle with Shredder...