You should focus your tanks target, you should use AoE's in most instances vs 3+ enemies. The game doesn't play like Classic WoW dungeons. You don't usually need to CC anything, running out of mana is usually a non-issue, taking on more than 1 pack isn't asking for a wipe, runners getting away grabbing multiple packs of mobs isn't going to happen, the tank shouldn't be losing aggro easily. It's a zergfest and an AoE one where applicable.