Once you enter the room from the right side, walk up to the Venus Weed as it begins its initial bloom animation. Once you see the white flower bud appear at the top, throw 3 Holy Waters on the ground at approximately where its root is. Then, quickly activate an Antivenom to give yourself invincibility frames and walk to the left until Alucard's sprite overlaps the middle portion of the Venus Weed and throw a Flame star. The damage from the Holy Waters and the Flame star should be enough to deplete the Venus Weed's health. If performed correctly, the Venus Weed will die without granting Alucard any XP.