Literally depends on what you want to do with Arena combat here. To be honest, the community is very nearly dead here, or at least, it's gone AWOL which tends to happen when nothing exciting has occurred for a while. If you want to just fight and mess around, hash out whatever rules you like with your opponent and make a thread in the subforum, do whatever, there aren't really any strict guidelines on that. If you want a ranked fight, then agree to an established ruleset with your opponent and ask for a judge in the Arena Chat thread, I'll happily judge a fight for you if you want as all the others have dissapeared. 

We -tend- to use T1 Eden here, which is a balancing method for magical powers predominantly, where abilities can be 'prepped' which essentially involves charging an attack in some fashion before unleashing it in a secondary post (think solar beam). The only caveat being you can prep for as long as you want, and magical defences may also be prepped. Numerical superiority in prepping constitutes your ability overcoming your opponents in a direct head-to-head fashion, in that way diverse magical powers are balanced.

Most of the balance in this basically comes down to ensuring you and your opponent have characters that are roughly equal in strength or equipment. This is the most difficult thing to do usually, as it requires you to match-up themes or you have disparities between ordinary men with pistols and magical ninjas with flaming katanas, where the ordinary man with a gun still wins through merit of guns being OP as hell in this sort of setting.

Anyway, you can check out T1 Eden rules here. [url]http://www.roleplayerguild.com/topics/87170-t1-eden-era-and-t1-modern-5-different-takes/ooc#post-3062213][/url]

Other than that, the best way to learn how to -do- Arena fighting is just to read some fights. There are plenty of styles that people use, I'm more technical, I write actions exactly as I mean them to be interpreted with some fluff in between to give an idea of what my character is feeling or thinking, some people tend to go more down the fluff route without really describing their character's actions that closely and leave it up to the 'rule of cool', others are predominantly technical and believe fluff is pointless. It's really up to one's tastes.