This boss appears to be unbeatable at first. I died about a half-dozen times.
I have no idea how to defend against whatever he uses to hit so hard.
Eventually I realised I had to fight dirty.
I found the trick is to simply unleash to Storm Atronach on him and dodge behind a rock. A few curses and lightning bolts appear to be optional. He goes down surprisingly fast.
'Clever' sigs get old real fast - just like this one.