An older character in a group of newbies.

He is incredibly well balanced and has an incredibly high amount of luck, this makes it hard to balance the game as he can overcome most challenge thrown at him. He has over 500 xp compared to the other relatively new characters. All this makes an incredibly unbalanced game.

I am the Game Master for a game of three other people and I have 1 character who has simply become too powerful. I try to make it difficult to even the playing field but he can withstand anything I can throw at him. It is difficult to make a fair campaign because the other players are still behind him in experience so it either turns out too easy for him or too hard for the others. How do I balance this out so that it is still a fun and fair campaign for everyone.