So... Newb here. Have any general advice on writing for wrhg?
Dude, you already have an excellent wRHG. You don't need my help.
That said, my biggest piece of advice period, is go with an archetype or central theme and expand on it with abilities and skills to compliment it as well as logical weaknesses. From there just compare it with other gladiators so that it is not too powerful or weak and heed the advice of those who claim it is if they make logical points.
As for backstory, I got nothing really to help you here. Just go with something you want and try to be logical but not cliche and unoriginal unless you have a really good reason. It also helps if there is a logical reason for your character getting his or her abilities.
As for personality, do whatever but I usually start with a vanilla character (good, mean, evil, etc) then give them their own motivations, attitudes, quirks, and whatever would be the logical outcome of his backstory. Add enough of this and the character will get fairly complex fairly quickly.
For appearance, go with a logical appearance to suit the character's backstory, personality, and abilities.
For the demo, do whatever you want.
For bonus points, have everything tie together to make sense like having a weakness tied to your backstory such as a phobia etc.
Rule 1#: Write something you enjoy.
Disclaimer: This is all stuff I came to believe as of late way after I made my wRHG character.
Now if you want to talk specific archetypes or you have a vision of a character that you want made feasible, feel free to PM me or someone else who's willing to help. There's a lot of nice people willing to help and they probably have a better since of balance than me.
Edit: I just realized you were talking about writing in general and not characters. I feel like a total idiot now, but i'm still keeping this up in case someone else can get use from it. Your basic writing skills a very good by the way so you should be fine. Just try to learn from reading other people and take CnC.