Bloodlines G 10

An easy way to earn this achievement is to collect as many hearts as possible (99) by using the secret passage at the top of the stairs - watch the vid, but stop there - and using the holy water power-up weapon. You need to stand at one of the candle sticks in Dracula's throne room to collect the heart that you will need. Use the power-up consecutivly until he is dead.

But the easiest way is Drachen77's:

Just open the attic, go back and forth between the attic and regular floor and collect hearts up to 99. Get the Holy water and just hit Y. Two shots of Holy Rain will drop his first form and then two shots again on the demon form and,,,presto!