Arena Achilles

Hitting enemies in melee range with your shield stuns them. Use this to keep the enemy close and stuck in the middle of a fight.

Don't throw down Radiant Glory as a fight breaks out, take a hit or two to maximize the healing from it.

Use Combat Dodge to get into a better position in a fight and use the throw to get off nice damage. Make sure to hit the throw so you can use Combat Dodge again.

Save your ult for executing enemies or closing the gap on low health enemies. Even if you don't kill them with your ult, the extra distance could get you the kill.

I suggest staying in defensive form for your passive, however if you start to fall behind, going into offense form and playing like an assassin is a great way to catch up.

Skill level order doesn't matter too much for him, but I recommend finishing them in the following order.
Fatal Strike -> Shield of Achilles -> Radiant Glory -> Combat Dodge

This is what I do to go positive in every arena match and win most of them. It works well with Clash too. I have not tested this in other modes.