This boss is a DPS check, you must kill him as quickly as possible and ideally not long after he begins creating copies of himself. If you do not kill the boss within 45 seconds or so he will begin to spawn copies of himself. It starts with three copies and then multiplies fast. At 95 seconds you can have the boss and as many as fifteen copies attacking you, which would imply a set of three copies created every 10 seconds. Have the tanks grab the copies and hold aggro but no DPS should be attacking them as all DPS needs to focus on the original T'Haen. He also has a minor AE knockback that throws you about two feet, so be sure you're not standing at the edge of the platform.