Leveling a tank and healer. Seems like 2 packs at a time is the way to go. It's usually enough enemies to make AoE effective, not enough to have single target DPS feel insignificant/TP starved, and give the healer enough time to DPS a little and not lose their minds.

Then again I'm leveling War and Whm, the former has an effective 40% reduction at will or for a whole dungeon with their CDs, and the latter is swimming in heals.