Unfair Map Boss Situation I Encountered

My class is blood mage, I entered a Boss area on a map. It was the silverback guy. Easy, except he had that blue ring aura that drains mana and it was a tight arena. I never knew bosses could have that? Drains my mana, so blood mage kicks in full tilt. Killing myself and chugging life potions. Needless to say I didn't last long b4 the boss clipped me while I was low health. Splat. I 100% don't think that was a fair fight. Other than that, love the game.