To beat this boss really easy take two caster, with Mother Earth Rune and Magic Absorb Rune. Have them cast every other round giving each other the chance to get mp for another cast back. Have main tank, I used galleon and equipe him with a few mega potions. Reason for this you don't want to set off the magic barrier cast my the Mother Earth Rune. With this combo the boss went down really easy for me. My main character was 50 with the rest around 45 to lvl 48. I hope this helps you get Eresh a little easier