When Morchok uses his stomp ability he splits 500,000 damage between all enemies within 20 yards. To avoid casualties the group should stay relatively close to eachother and prepare an AoE heal. The two closest targets take a double share of the damage, so needless to say they need to be proper healed before the stomp lands.
When Morchok conjures an explosive crystal (Twilight Orb) the group should hug the crystal before it detonates, to decrease its damage
When Morchok casts Falling Fragments, stalagmites will erupt from the ground and do 15,000 damage each. You can not channel spells behind them, seeing as they are obstacles
There are also a few spells you should know about before the fight, which are:
Black Blood of the Earth: Morchok channels the blood of the earth and inflicts 5000 Nature damage to nearby enemies, increasing Nature damage taken by 10% every sec. Stacks up to 20 times.
Fury: Morchok enrages, decreasing the time between his attacks by 50% and increases his damage by 20% for until cancelled.