First I'd like to share my view on AoE kills vs 1-after-1 kills:

AoE kills may net each player more damage per skill used, but when the damage dealt on each individual mob is checked, it won't be quicker than a series of focused single target damage skills.
It means, 1-after-1 can reduce the overall damage dealt to the tank, in case no Sleep is used; Whereas AoE kills will have the tank reduce its HP throughout the fight with the full damage from all mobs.