This boss is a DPS check, you must kill him as quickly as possible and ideally not long after he begins creating copies he also has a minor AE knockback that throws you about two feet at the most, so be sure you're not standing at the edge of the platform
Just have the tanks grab the copies and hold aggro but do not kill them. All DPS should keep working on the original T'Haen the entire time.
If you do not kill the boss in 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).