**Stop playing manual hack-and-slay, start role playing!**

- You could shift your sessions to reward good role playing and creative solutions more, and put less focus on the fights. Create interesting and challenging tactical solutions instead of throwing foe after foe at them.

- You could punish characters that are not well justified in-game. There are enough in-game reasons to take away crucial parts of an optimized build. Have the character suffer through an odyssey to try and get them back. 

- Just don´t allow every possible customization in your setting. Make the players earn their desired (multi)classes/ feats and boni. You could require them to find trainers etc and make it actually challenging to even get to the character-build they are after.

-  Let him have fun with his optimizations, but just let him lag behind on character level. This is partially an out-of-game solution, but when everyone agrees with it that it would be the most fun for everyone, this could balance things out. Why not have this exceptionally strong lvl 8 char run with the lvl 10 group?