No the last boss is easy... he just takes SOO long.
The Dragon is just really really annoying and tough once he hits you with mind control because after that he sends rocks to hit you meaning you have to find out what "Shield" has been reassigned onto.
Disclaimer: I haven't played it in months, myself (except for getting a steam summer camp achievement). I did the campaign singleplayer since it was bugged and didn't work multiplayer back then.
He actually hurt you? How did that happen?
I just teleported to his stomach and then stabbed him repeatedly until he died. He couldn't figure out how to hit me.
I don't remember how I beat the last boss, but I'm pretty sure it involved nullifying everything he cast. The other part of it was probably hitting him with the most overpowered beam spell (one of the arcane steam lightning combos, QFQFASA or QFQFQFAS or something. That probably doesn't work as well now.). Fighting imprisoned!Grimnir wasn't that hard either. I froze him and hit him with cold beams for massive damage. Nullify could take out the ghosts, or shields could block their shots.
The hardest part of the game was the part where you get sent to the islands to fight all those mages (after meeting Grimnir, when he concludes that you must be tested before facing him), especially the ones that go LOL WATER BEAM FALL TO YOUR DEATH OVER AND OVER at the very beginning. There were staves which had an emergency teleport feature to save you from falling to your death, but the most likely place you would find one was those very same mages with those water beams, and you would have had to have known to grab one of their staves before you killed them all, since there's no chance to pick up loot after they're all dead. Grab one and the rest of the islands become much safer, however.
Edit: Chrome neglected to point out that 'singelplayer' was not spelled correctly, but was more than happy to point out that 'singleplayer' wasn't a word either once I had corrected my spelling. Spellcheck, what do you mean spellcheck isn't a word? I reject your reality and substitute my own!