Welcome to Old Clockworks's Boss Mission, Showtime. If you've been following our guide, there will be only 1 Gem Stone that we recommend grabbing here that you haven't already found.
Interact with the clock gate to place both hands and the rotor you found back in their proper place, allowing you to now operate the clock. The catch is that you have to set it to the proper time to open the way. But if you happened to peek through the hole in the wall on the right, you might have noticed three numbers 7, 3, and 0. As you might have guessed, that means 7:30, so go ahead and set the clock to that time to by using your vacuum on the rotor. Once you've set it correctly, the gate will lift out of the way revealing a door that leads into the Movements Room. Inside, head left and use your vacuum through the window to retrieve the mansion's 13th gem.
Now the section of ground you're on is actually a giant lift. To begin raising it, use your vacuum on a rotor to the left.. You'll soon be raised beyond the point that you can reach it, but there will be another rotor on the right you can reach to continue your ascent, followed by one more on the left. Using all three, you'll be able to reach the door above that leads into the Belfry. Now just head toward the ladder to find yourself launched to the boss's arena, which takes place on a giant clock. But even though it is a boss battle, you'll actually spend most of it battling the more conventional enemies you've fought before. Because in this battle, you'll have to take on a new wave of enemies for each hour of the clock, or 12 in total. Now the catch is that if you don't completely clear out all the enemies in each wave before the minute-hand circles around the clock, you'll have to repeat that wave from the start. Fortunately, you can actually use your Poltergust to rewind the clockhands buying yourself more time. Conversely, you can speed it up too, though there's really no reason you'd want to do this.
As for the waves of enemies, your usual tactics should work fine for the most part. But if you're quick on the draw with the strobe light, you should be able to stun or take down many of the enemies almost immediately at the start of a round since they all appear from same place. Between rounds, make sure to stand on the outer edge of the arena to avoid being taken out by the fast-moving clockhands--also make sure to avoid them entirely when they glow red.
After every fourth round, the Poltergeist will make a proper appearance. Keep on the move to avoid his dash attacks, then get ready to stun and vacuum him after the fourth one when he temporarily KOs himself to inflict some pain. Repeat the process each time he appears to capture him for good and reveal a piece of the Dark Moon that's yours for the taking.
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