Beat the Shadow, Pazuzu, Minotaur Lv2, Legion (corpse) bosses, and both forms of Death.
Death is really pathetic in this Castlevania, since by the time I got to him, my defense was so high, he could only do 1 damage per hit to me.
I obtained the Griffin's Wing relic, which allows me to super jump, so I can now explore most of both castles, other then some locked doors. Though I did find the Skill key, which allows me to unlock Skull doors.
I'm currently in the Entrance, in Castle A, I'm level 43, gained a lot of levels this time.