Jitters Caffeine said:
I've been in that position before. But, it was I who was the overpowered/unstoppable protagonist in this case. I've hosted a few RP Threads beforehand, thus had gone over the top without even realizing it.
At first people went along with it, liking my style... however it sort of worn out making it harder for them to get any of the action so I was asked to tone it down. Since I respected them thus they were pretty much my friends, I did so (sadly it caused my character to die for a good cause.)
Anyhow, since he is considered your friend, perhaps talk to him and ask politely if he'll work with you guys as a team. Asking for him to stop altogether might send the wrong message, so instead request if you guys can be more involved within the fights if not the whole game. If he is still being unreasonable with the new race/class he's chosen, then you guys can vote to see if he'll have to wait longer or have particular consequences in the game for being so powerful. After all, you can't do a huge combo and expect to get right back up your feet. If you're still in a tight spot, please feel free to contact me in a PM. I'll be more then happy to listen and help you out. Good luck in your D&D game, wishing you the best to come your way.