So how is this Arena RP stuff supposed to work?
so uh eh first you gotta find yourself a partn'r, preferably someone that you find agreeable and can get along with, so as to in turn, ease resolving the inevitable disagreements; and also close to your skill level in understanding of RP fighting in particular, and combat/physics in general. Likely improbable, seeing as RPGuild's arena has turned to ghost town, but you can always drag in someone else into these confines or go seeking in different places.
then you got your character sheets and whatnot. very important.
The character sheet is reference for every action attempted in-character - if there's a question along the lines of "can he outspeed/break through/tank out/jump over/melt down/freeze/smash/swallow what they bite", then the go to method to finding an answer is peeking into the character sheets and comparing the respective character capabilities: if P1 can punch like a sledgehammer and P2's face is likened to a brick in their character sheet, then something's gonna be brick red alright.
For this reason it's important to be concrete and plentiful with detail; the two prominent philosophies are either providing exact physical measurements - 100 mp/h sprint speed, 10 ton deadlift, 2000 celcius firebolt, etc. - or, using comparative feats - strong enough to lift and toss cars, as durable as a tank hull, breaching supersonic speed. Important to note, it's not only the magnitude of your characters capabilities that should be included - the "how much" - but also the mechanics and inner workings of any ability/power/aspect they have - the simple "how". For example, a searing 5000 celcius plasma bolt might do heavy damage to unprotected material, but due to its quick dissipation, have negligible impact on an insulated target; whereas as sticking ball of napalm, burning at 500 degrees, might adhere for long enough to seep its heat through the insulation and bake whatever's beneath.
finally! after making sure you wanna fight and your characters are of comparable power, you actually do the thing
the same general notion applies here - plentiful and thorough detailing enriches the experience, as the more detail you provide to your actions, the deeper the possible degree of interaction is; for ever property you describe, that becomes a property that can be interacted with. The speed, the angle, the timing and the degree of commitment to an action; your character's positioning in every key moment in the fight; the information your character currently does know and can operate with; etc.
However, it's all up to you and your partner to decide how complex you want your fight to be, adjusting by omitting or including various aspects of the action - the beauty of RP fighting is that it has minimal restrictions, by virtue of every rule and mechanic being player regulated.
That should conclude a basic introduction to the general nuance of RP fighting; all else is best pursued in raw experience.