There's little solid, or for that matter, flat ground for the Mutant Rancor to walk on. The Mutant Rancor would definitely die if it fell into lava, because I doubt it could get out. Thus, Boba Fett and Jango Fett win by default unless they get stupid (not using their jetpacks) and the Mutant Rancor gets lucky (being able to get at the Fetts, I don't know how easily the Rancor can move through the facility, if at all).