Here's one I've just deleveled. May not be perfect, but basic principle:
Has two weapons, and has shield (the wonders of the weapon-shield) Knocks people prone when he moves them, which is most of the time.
Can knock prone on opportunity attack, and on combat challenge. Doesn't have much to capitalise on this, other than the fact it wastes enemy actions.
Not sure exactly what the arena thing entails, so not quite sure how suitable this guy is.
EDIT: Edited to change a couple of things so the deleveled version works better.
Note: while both attacks in Driving Flurry can slide 2, only the one which uses the spear can knock prone.
