Golden Vastus is not super fast next to the fire pit...
He actually died in it when I was Mata Nui.
I also figured out away to keep Gelu and others from healing. Just go up to them and jump to the side back and forth. Heh, that's funny, I never fight in Arena Magna, due to the lack of the fire pit.
To beat Stronius, I gave him 2-4, one time 8 regular hits all at once, next to the fire pit.
Gelu is tough...but like you said, you have to keep him from healing and then use thornax. I have gotten a few light attacks on him though.
