The difference if you attack Damian with melee weapons is that he will then summon some random minions to help (from 4 to 8, depending on the difficulty setting). The only one you should have to be too concerned about, though, is the necromancer, who has an effective fireball spell. Other than that, enough healing / restoration potions will see you through as long as Damian can not kill you in a couple strong hits. Strength / Agility / Vitality / Speed potions would help, as well.
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