My theory for what it's worth:
Jamie will fight for Tyrion, just seems to fit with him being trained up by Bronn. Seems to be the only one who so far hasn't betrayed him either.
I think Aria will be the one to eventually kill The Mountain so I don't think he'll be the one he's up against. Can't think of who else it could be though, guessing it'll be someone he doesn't want to kill. Would've said it's the giant woman he used to hang around with but she's half way around the world. Or maybe it'll be Bronn? Teacher and student battling it out to the death seems right up Martin's street.