Warlocks have AoE Fear at lvl 40, so if you can wait until then, that should work fine for "oh crap" moments with extra adds. With multiple locks, you can make a cast sequence on each of your locks so you can do 2x in a row if necessary.
Code:
/castsequence howl of terror, 
/castsequence ,howl of terror
and it should help a bunch. You could also then use focus to target any stragglers, or just fear a couple after you are ready to burn those adds down.

I want to be able to set up focus on each of my slaves separately, so I can CC them all effectively. If you have multiple versions of CC it gets tricky (sheep/fear, etc) you could also add modifiers in, so one keypress could focus one character on a particular target, then tab to another one, hold a modifier, and press same button, and another character focuses that one, etc. If you also use your main spells as /cast [target=party1target] like I do, that also frees up the regular /target. That basically gives you 2 targets, so you could have one lock fear the focus, and one fear the target.