Well, the game has flaws at high levels.
I bet the rest of the party is similarly powerful. If they're not, tell the powergaming player to start lending his aid to the other players. Then either start a new campaign or switch to another game that doesn't have those particular issues (maybe playtest D&D Next). Other games just have different issues, though.
I suggest you avoid adversarial GMing. The player clearly wants a character who is hard to kill, so great! Don't try to kill him. Focus on the other PCs, and maybe occasionally throw in a rogue 12/sorcerer 4 who uses casts invisibility (but still moves silently and hides while invisible, just in case someone has true seeing), then casts silent True Strike and sneak attacks the monk with a +4 spell-storing short sword that immediately casts vampiric touch (which his 20th level necromancer buddy put in the weapon for him). So you've got an attack bonus of +11 base, +6 Finesse, +4 magic weapon, +1 weapon focus, +20 true strike = +42, so he only misses on a 1 even if somehow the monk knows he's there. Then he hits for 1d6+4 plus 6d6 sneak attack plus 10d6, average of 61 damage.
Then turn invisible, retreat, and try again later.
But generally, it's a losing proposition to try and kill an overpowered PC, because the rest of the party ends up as collateral damage.
Oh, and ghost dragons work pretty well, with incorporeal touch attacks.
