I have no real idea about the main problem you are facing, but quick solution (by no means permanent) would be to face the other opposite side/against a wall and cast. The main problem with Teron is that you all will gather in one small area and thus your FPS will definitely dip with all the casting occuring.

However, with your spec, it doesnt seem that it should be a problem unless there is something else running at the background or that you have a lot of unnecessary mods.

That is ONE boss I'd hate to even have a momentary spike :S Sucks to be playing from Malaysia.