This isn't actually true by the way - even inexperienced teams should do fine. This is our damage breakdown to the adds this week applying the zerg-tactic:
https://www.warcraftlogs.com/reports...one&target=209
As you can see, only me, a warrior and a warlock were really trying to target the adds. Three people out of 15. The rest of them are pretty much pure AOE/cleave (chain lightning, ignite, disease etc). Their setup has double the hunters that absolutely annihilate this fight.
The one thing we DID do, was have our rerolling mage (who hasn't bothered learning how to play fire cos rerolling / returning player anyway) be arcane, and cast slow on every single add that spawned. 15 seconds instant cast slow is the best one available, and having it cast on everything means it's piss easy.