This is a roleplaying pronblem: not a mechanics issue. He has done nothing wrong mechanically.
It becomes a mechanic issue when no one is capable of standing up to his character. His AC for his character and his humunculi are nearly, if not completely, beyond what we can hit. Within a matter of turns he could kill the whole party. Where is the fun of playing with someone who strongarms the other players and there is no other recourse than some sort of nuke? When the Gm throws powerful creatures our way and that doesn't even phase him, but it almost certainly hurts us, what other recourse but extreme recourse is there?
I mean, honestly, if this is what D&D has become, then this is ridiculous. I was brought in in an attempt to make the game fun for the other players. It's not fun with this character, but I don't want to tell the other players and the GM that it's a lost cause. I'm at a loss as to what I can do to correct this situation. He's upset a powerful cleric, but if the big bads of the monster manuals can't phase the character, what else can be done to at least curb him a bit? The one humunculi idea is a start, but I'm not sure how that will fly. If I can enact something in-game to curb it, then good.
While I really just want to see this character die, I can accept an in-game consequence for his latest actions that will put him into an equal footing with the other players.