I found him to be either, incredible hard, or incredibly easy, depending on whether or not you had some way of becoming invincible. The first time I played the game I didnt take the time to play the card game and the only hero drink was wasted by one of my characters who were confused in a previous battle. So when I faught him I was completly obliterated.

Being wiser the second time around, I played the card game, aquired the gilgamesh card and destroyed him in the end. I mean I still had to worry about my invincibilty to wear off, but I mean, otherwise it was smooth sailing like you couldn't believe.