The Scenario
Greater Sci-Fi combatant
Hm.
I'm not sure what to make of this one, given how difficult it's going to be determining Rayquaza's abilities. It has one scene in Emerald where it calms down Kyogre and Groudon, but is otherwise featless. Meanwhile Kyogre and Groudon have the potential to flood the planet or cause extended droughts, respectively, putting Rayquaza roughly on that level of power with the ability to calm the weather instead. I haven't seen the anime or read the manga where it appears, so I've got nothing there.
Ridley, on the other hand, can survive ship self-destructs and regenerate by eating meat, which I'm assuming aren't available. Defensively, even his adolescent form is immune to bullets and is only seems to be fazed by plasma, while his adult form can temporarily shrug off plasma attacks, but not larger explosives. He breathes fire, which Rayquaza's Dragon typing has resistance to, but Ridley also wrecks a mountain in the Metroid manga. Otherwise, Ridley has few other notable attacks aside from claws and his tail. He has a chameleon-like ability to disguise himself and be almost invisible, and his screeching sends animals into a frenzied bloodrage; not sure how exactly that will work, but could be advantageous .
Meta-Ridley is a bit different, with plasma breath, bombs, missiles, a lot more armor and in Omega form, Phazon powers. Not too many feats aside from destroying statues and bending metal bars, but he's definitely more powerful than standard Ridley. Phazon in particular gives him a good edge.
As for the battlefield, Ridley isn't too bothered by being near magma and Rayquaza seems to deal with Groudon well enough. The volcano shouldn't affect either of them too adversely.