This is obtained by killing the Arena III boss on Wave 10. Not the easiest of arenas to clear, but the boss is overall pretty easy.
He comes out of the top and bottom of the arena and joins in the middle. This process is easily telegraphed by some breakage in the floor/ceiling and can easily be avoided. It doesn't matter which direction you go, so just get clear of the breakage.
He will then proceed to shoot two orbs to damage you from whatever side of him you're on. I utilized the grenade launcher LT attack to destroy the orbs and damage him in the process. Otherwise, they're pretty slow-moving and easy to avoid if you're using a different weapon.