Onyxia is one of the first raid bosses created by Blizzard and as stupid, as she smells, is fighting her. First you'll need 35 heroes, which should know each other. If it should be a random group, you'll need about 487 heroes, so just don't try it, because 238 of those are going to stop at the entry crying:
"Prequest? Hu? What Prequest?"