I've never killed Mosshorn coz of the ridiculous drop, but all of the others are pretty easy, for exemple my ls can take the Big Buffalo down in 7 minutes and Dodore in 3 BR once the minions are dead. But I wouldn't call it zerg, actually you need to be trained and to have a party that can cooperate well to achieve that and those fights are really fun, definitely better then doing them in 15 people doing random damage.