You don't need to stand in circles, rather shield and stack.
The DPS check is mostly caused by the need to dodge the middle add (random attacker); you can bait the fireball by standing far and move when he prepares to spit it and dodge the cleave (he will keep alternating the 2). If people get hit they get a damage dealt down on them making it harder to kill the adds on time.
In HM kill him first to avoid the Damage dealt down debuff on people and you should be fine