Run to Arok and stand in front of him, but turned slightly to his right. Hit him with weapon #2 when he is in the air, this is better if he is in his highest position, then run quickly forwards ([Shift] + [Cursor Up]. You will pass him by and he will miss his punch.
You will end up behind him, from where you can hit him safely. In three or four times you can kill him very easily. Run quickly to the place in which he appears.