In the castle, there is a glitch where you can make the Gatling Gun appear without allowing the red monk to operate it.

First, progress normally and let the red monk escape. Once you return to the turret room, he will be operating the gatling gun. Immediate leave the room and return to the main hall with the merchant and save.

Load the game and re-enter the turret room. You need to chase the red monk again until he returns to the turret room. When you return to the turret room, the turret will be raised, but the red monk will be standing outside of the turret ring! Without the turret on his side, finish him with ease!