If your stuck on playing 5, I would go with 5x Shaman. When you have to heal your entire force is on defensive, but one swift heal and your party is at full hp.

There is no time to manage non instant single target heals combined with taking out targets. It will just never happen.

Furthermore the real question is how high do you want to take it? The core weakness a completely Multi-boxed team encounters is not having every enemy covered, it plagues me as is.

2 Res capable opponents forces me to play toggle just to keep them in combat leaving me 0 time to actually take out a target. I can only imagine the nightmare that would be if the enemy has 3 healer's attempting to res and trying to stop that from happening playing all 5. It goes beyond what class is best for Arena's and plays into a simple game of chess. Where you are at a distinct disadvantage.

I really hope my videos were not misleading - I've multi-boxed for a very long time, and I've always done it in a cutting edge format be it PvE or PvP in many different MMO's. It's not always roses