To answer the OP's question, yes, you'll probably have to either use multiple flashes in a row or break up your combo between the different enemies to maintain a high enough threat level on each one. Assuming you have good gear, mitigate damage via cooldowns and the healer isn't overhealing, then a few attacks here and there should be sufficient after the initial pull to maintain aggro. But not every healer will be careful so you may have to fight some of them.

Also keep in mind that a well placed Sleep can often do wonders if the party isn't using AoE on the enemies. It speeds up the killing of the other enemies and nullifies the threat issue for 30+15 seconds. Very useful indeed.