Thanks for the advice so far.

As far as AOE Def Down, I have Uugo maxed out and I've almost got Dhukk the Pierced at 60 (but more than a few potions short to ascend all the way). I pulled Genzin recently, but don't know how to use him yet.

If I keep Arbiter and Abbess, and then add in one of the above, what type of champ should I bring in to finish the group? 

Genzin would fit in quite well - he can do the AOE DEF down, and abbess finish off with her A3, and he can also do a single-target nuke to get through unkillable. Just be aware that since his A3 ignores shield, it also doesn't strip shield, which means if he doesn't finish the target off, it'll be harder to kill than you would otherwise expect.

The last piece that would really be helpful is someone with an AOE TM manipulation. You want to put Abbess and Genzin in pure damage gear, but in order to do that, you'll probably need to reduce the TM of their team so that they can act first.

Have you got anyone with good AOE TM, or perhaps even AOE stun or sleep? Rhazin would be the "easiest" to get since it's fuseable, but there are plenty of others (Nethril and Big'un come to mind).

A Very Classical Arena set up would run a Speed Buffing Hero.

Here is an example: 

i

Apothecary -----> Turn Meter Fill + Increase Speed Buff

Arbiter ----------> Turn Meter Fill + Increase Attack Buff

Warmaiden -----> AOE Def Down

Foli -------------> AOE Nuke

Your Abbess would replace my Foli.

Your Uugo, Dhukk, or Genzin would replace my Warmaiden

Your Arbiter would replace my Arbiter. lol

? - Not sure what options you have, but you might have something which can replace my Apothecary.