The Vampire Bat is the first boss and serves as an introduction to boss mechanics. It flies across the top of the screen, then swoops down towards Simon. Its weak point is its body. The best strategy is to stand in the center of the room. As the bat swoops down, whip it once or twice, then move out of the way to avoid collision. Repeat this process. If you have the Axe sub-weapon, it can be very effective here, as it flies upwards in an arc, hitting the bat as it flies high. The Dagger is also decent for consistent damage. Focus on dodging its body and striking when it's vulnerable. It takes several hits, so be patient and avoid taking unnecessary damage. Its pattern is predictable, making it a good boss to practice your timing.
