In Onyxia's Lair when she lands and starts phase three have everyone, except the main tank(s) mind you, climb/jump up the cracks along the walls. If successfully done they should be a few feet above the ground along side the top part of the wall. When Onyxia fears the people that are on top of the wall they just run around the outline of it and not down into the middle of the room where they would take fire damage and have a chance to aggro whelps.