When you're finished with them a door will open. So head on in and continue to a room with a mechanism with a spoke missing (#1). In the next room there's Velvet waiting for you (you'll hear her voice if you approach the door). But - first things first - finish with Sir William. Walk ahead and turn left for final meeting. The fight should not be a problem: use your minions (#2) and support them with your weapons and spells. Avoid his hits and keep an eye on him as he'd use his magic and he'll teleport to different places in the room - so keep following him. When a group of these red guys gather around him - give him the final shot.