It just SEEMS that strong. If you're not playing on easy, you should first ensure you won't catch cold during long fight (I personally create two fires with Immolate around the big blob near the right edge of the area - to run around it so that Doppelganger can't hit me).
The thing you should know is that your Psionic Blast seems to damage the more the closer you are to your target, and (it seems to me) the faster it's launched. When you move and cast or shoot, your velocity vector is added to the projectile's velocity vector. So, if you charge towards him for a brief moment and blast near him, you'll deal more damage.
So, the only thing is beware his chilly blast - it makes you very vulnerable to the next attacks (if he doesn't miss , hehe). And dodge, dodge... What else can I say?
EDIT: I'm very used to Static control system in Notrium, not Relative one. Maybe the fight gets a bit different when playing with Relative system, I don't know.