It's either Luffy beat up Moria to get back his shadow, or Luffy beat up Oz, and get back his shadow. I feel that Luffy's not the type to go 8v1 (honest question: has something happened like that before?). Iirc, Luffy always says 'I'll take care of that one!' and he does it. He wants to be the pirate king remember? He needs to be the most powerful man so he can protect his nakama. He'll do it alone. Either Moria or Oz. -nods- I'd rather it be Oz, more interesting!
Oz seems rather overpowered, but Luffy can do ittttttttttt!
If it's Luffy vs Moria, maybe it'll be the rest of the team vs Oz to keep him from interfering with L v M's fight.
(PS: Please pardon any untruths in this small paragraph, it's been years since I first read one piece, can't expect me to remember everything )