Shaman if horde, mages if alliance. Shamans just have so much utility for a boxer its not even close. If you are not stuck on having all the same you could toss one mage into the mix since both are ranged dps. Granted syncing of similar abilities wouldnt be exactly 1:1 in terms of timing/damage but it wouldnt be impossible. Benefit would be slowed enemies, conjured food/water, and portals for your team.

IE:
Lightning bolt = Frostbolt and/or fireball /castsequence macro
Flame shock/frost shock = fireblast
Poly = hex (round robin)
rooting totem = frost nova (round robin)

AE/AOE is a little tougher but if you drove from the mage then Blizzard + chain lightning wouldnt be too bad to be honest.