While others have said that AoE is the way to go on large pulls (technically 3+ mobs for every class due to group AoE synergies), I feel they are not being specific enough.

If you have a single target spell/combo you have to weave in to keep buffs up, or a spell with a higher potency on the first target:
It's an efficiency gain if you hit the HIGHEST health mob first. Because ideally, the AoE kills all the mobs at the same time. Every mob that dies early is an AoE DPS loss. (of 180 potency per F2 per early mob death if you're talking BLM).
So to keep GROUP DPS up, when you have single target stuff you have to do, switch to highest health target. ESPECIALLY if you're with MNK or BLM.
But the rule works with every DPS class' AoE rotation it's just more noticeable with MNK and BLM.