That paintwork is really good! He certainly looks the part for the arena. I like the troll in the background too.
My only issue is my own OCD. I don't mind alternative models, but I feel like the bases should all be real AR bases for playing AR. Mismatching bases just feels... wrong.
My main concern with the rules is the thing about giving him attack 8. It seems like an extremely homebrew kinda thing to do - to make a model that has stats higher than anything in the real game. Like turning up the dial to 11. Nothing wrong with it in principle, he doesn't seem unbalanced, it's just one of those signals that it's fanfiction rather than real fiction, if you know what I mean.
What matters is that you have fun with it and that it makes sense in your games, of course. I think if I wanted to use him in my own games, I would go a slightly different route.
I agree that since he is standing on two legs and wielding a weapon, he doesn't look like a beast. Just a titan. And I would probably give him two stages. Since he is pretty big. I think I would give him more normal stats, like ATK 6 and ARM 2, but some kind of interesting special rule to represent his big chain hammer thing. Like maybe if he does a power attack, all his damage tree boxes do +1 damage. (And then rework his damage tree appropriately, obviously.) He kinda seems like a berserker, so I could see him being good at power attacking.
Just some random brainstorming.