Prof > Firefight (I'm assuming the real Firefight and not Megan)- I don't think Firefight has any way to deal with Prof's forcefields. Prof would probably just smash him in one within a few seconds and be done with it.
Wingflare > Loophole- I'm not sure how Wingflare would actually get the kill, but she can stop Loophole from moving, so there's not much Loophole can really do. I suppose Wingflare could just let her starve to death in the most boring cage match ever.
Jax > Regalia- Unless the cage is surrounded by water Jax would have Regalia dead before her powers could come into play.
Nightwielder > Lifeforce- Eventually Lifeforce would run out of people to heal him. His only shot is guessing Nightwielder's weakness, and the chances of him doing that and being able to take advantage are slim. This would likely be very long and very gruesome to watch.
Steelslayer > Obliteration- I was tempted to say draw, because at the end of the books, I don't think they are at a point where they could kill each other yet. That said, if they had to keep fighting until one died, I think David eventually figures out Obliteration's weakness or another way to kill him.