The trick behind being able to jump and reach Crimson Eye is all about utilizing the momentum and gravity. The boss fight takes place when you step onto the elevator and Crimson Eye locks you inside with it. The elevator then accelerates up and down throughout the fight, so basically... When the elevator begins to drop, your jump height is increased due to gravity. Then you can jump up and reach him. Likewise when the elevator is accelerating upward, your jumping ability is diminished due to the force of the elevator careening upward.