Baal's Spiral Missile

I need to know how it does damage. Whenever my sorc gets hit by it (45% damage reduce + 56% energy shield for 75% DR), it takes out half or ALL mana, and 50% of my life. I know it takes out mana and life, but it shouldnt do several hundred damage worth, 500 life and 900 mana lost is impossible, only diablo can do that type of damage with his firestorm, but only at close range. According to the page its a physical attack that takes out 50% life/mana (dunno about life), but no move is non resistable, except berzerk. So that means I should lose 250 life, and 700 mana, so either its a special type of attack, or it can hit several times in 1 passing. It has ITD, extremely quick, and kills any merc in 1 hit.