Well, I have a few thoughts. first though, you only listed 5 stats, 12/17/17/14/20 which if you rolled 5d6b3 you shouldn't get a 20 with a half elf. Could you clarify what your starting rolls were please?

Regardless of that I would suggest Cha>Int>Con>Dex>Str>Will for your stats, whatever they may be.

I would suggest you take Combat Expertise and Improved Trip or Improved Disarm, assuming your strength score is above 11. If you don't have any strength bonus, don't take improved trip or improved disarm. This gives you agency in battle while your allies can do the heavier combat.

Have a whip as your primary weapon and a shortsword or rapier as your backup weapon.

This should prove quite useful even in an arena. It gives you the opportunity to win battles with flair and pazaz as you trip your enemies and cut their throats or rip their weapons from their hands and leave them to defend themselves without a weapon.