4 SL/SL warlocks and a 1 healer is insane in the arena.

My next group is going to be 4 Locks. Why?

Because you can DPS and LoS. I cant think of a multibox that can put out higher DPS while on the move and have the synergy of stacked DOTS. This group allows for you to use LoS and takes away teams ability to run from your DPS. Throw in a healer and this team is nasty good. I think only mass dispell has a chance of removing a fully dotted target...any other attempt to dispell the DOTS will end in a dead target. That many DOTs ticks uber hard.

Dots proc'ing multiple Nightfall used to kill another target = priceless. This was the case when I fough this group compisition.

It might be a little boring since the DoT dmg is not fun to watch, but having a rezzer and Soul Stone X 4 would make efficient...not that Locks die while leveling...

Some challenges I can think of right away though,
Pet Control, Cloth (low armor), Pure Magic (Rogues and Feral druids are going to give you a run for your money).

I think a Priest or Resto Shammy would make a great healer for the group. The healer is the weakness of this team, so the Mail and totems would help with survivability but a priest has HoTs and shield.