Multiboxing is allowed on retail, and therefore allowed here too.
I don't have a problem against multiboxing, as everyone can do it if they have the time and strong enough pc for that. (tho that dudu guy just bought all of his chars from a multiboxer who quit the server).
A multiboxer in theory is less powerful than same amount of players playing their chars, but in reality, a multiboxer can 1 shot most players 1 by 1, while regular players, even when communicating with voice chat can't make sure all of them attacking the same target, therefore spreading damage and giving the multiboxer more time.
In addition to all that, a druid multiboxer who use Starfall, an instant aoe which cannot be interrupted or dispelled, will kill most players around him without even targeting them.
On a single boomkin that use starfall, the usual tactic is either kill him fast, OR MC him, making him damage his allies instead.
On a multiboxer which use starfall, I have no idea what to do except using defensive cd and hoping enough of you survive to do enough damage on him when starfall is over.