Obvious, you can't. you need a black belt or higher (Fire Card Jitsu) to defeat the sensei.
You beat the sensai
To defeat him you have to earn all the belts by versing other penguins. Talk to the sensei then click earn your belts. When you have the black belt you can defeat the sensai.
To beat the sensei,you have to be a black belt. You constantly have to try again,and again to beat him,because he gets weaker every time you lose.
You go to sensai and beat him to become a ninja!
you must have you're black belt and beat the sensai
You can't you need to keep on fighting people on Club Penguin then when you get to black belt fight Sensai and if you win you get a suit, if you're a member you can turn invisible, oh and there's a secret room you can go in with black belt. Hope I helped :)
first get your black belt then play him eleven times.you will lose all the eleven times. then he becomes playable.play him two times.you will win
You need to keep playing Card-Jitsu until you earn up to the black belt. Then, challenge the Sensei and if you beat him, he will make you ninja.
Become a black belt
Go to the Dojo, play Competition Mode (well, you could play on the mats, but competition mode is faster) earn all your belts. When you get your black belt, challenge the Sensei, it takes a while to defeat the Sensei, you need to play him a few times and then he will get weak. Go to the Server Mammoth and dance in the middle of the dojo for 30 miniutes or so anf u will get your black belt then go to challenge the sensei
in the side of the dojo but, you have to be a black belt and defeat sensei
with club penguin trainer 2