If you stay within the central circle Barnak cannot use his magic. If he forces you into the main arena he will attack you with whirlwinds, earthquakes, lightening and fireballs.
As he approaches slash, when he backs off use your magic (the electric spear is the fastest and my favourite). Do not let him get too close or he will jump on you.
I found the best solution was to stay in one place, turn to face him when he materialises, slash and fire off some magic. If you get it just right you can defeat him in 8 or 9 strikes.