You get this for beating Shredder at the end of the game, so it's unmissable.
In order to beat Shredder, you must use your shell kickers to win and as long as you know what you're doing it won't be very hard. If you're having problems, keep switching out your turtles because every time another one comes in it gets a few seconds of invulnerability.
For each stage of the fight, Shredder has many moves that he can do, like flying around the arena, and calling in more baddies. You can only attack him when he's tired, he's boasting, or when it looks like his arm malfunctioned since every other time he will dodge everything.
For the first stage of the fight, you must knock Shredder into one of the sides of the arena. Second, you must knock him upwards into the magnet that moves above you. Third you must knock him straight down. After that, it's over.