wait you honestly want something I have to offer, I honestly thought you would just ignore my post.
I would start with a glabrezu strip the magic of it increase the armour class to hard to hurt at all level and add a troll like healing factor.
make him resistant to everything by psionic damage which he take double damage from he also can't heal from it.
can;t swim but can walk or run under water much faster than most with a base speed of 45
possibly add a barbarian like rage for when he hits half health if the pc's are doing to well
back him up with every crab or crab like thing you can get or at least sufficent for a good encounter.
is most likely after the local MacGuffin for power or at least the ability to improve his life in some fashion as he was always the dimest of his associates so to a degree the one lest able to use it.
I can do five more but I am not use to monster crafting.
