In worst case scenario, remember that he have only one attack per round ...
So if you have REALLY LOT OF TIME ... and patience ... and dont mind geting it all ridiculous ... you can easily defeat him by sheer numbers.
He kills one of yours > other of yours will "Help" them alive > another member of your party attacks > and repeat.
Its a long and tedious process, but its litteraly impossible to loose this way, even 1v2 is inevidable win for you.
