If you need to buy a little extra time when the adds come, you could always try jumping from the elevated position you're fighting on, down to the lower floor. Once the adds are near you, they would then have to turn around and run back down the ramp.

If you have DoTs, you could get a new round of DoTs to finish off the boss before it runs down the ramp as well.

If you have good control of your alts, you could just drop the four alts down long enough to pick up the adds, and then jump down with the main, with enough aggro do dps all 3 down.