You might've seen me send this earlier but I'm re-commenting to say that I've figured it out and this is a guide to officially "beat" the game. Spoilers ig lol
So to begin with, you're going to need every perk: Bellow, Drain, and Peer. Both the Peer window and Drain window are movable, and there are indicators of where they need to be moved (Drain has a slight chip on the side of it, and Peer's eye). You can open shop to move them around easily. When moved to the correct locations (You'll know it when you see it), the windows should now say Peer! & Drain! respectively. You'll also notice that Peer! looks different.
When the previous steps are done, stand inside of Peer!'s eye and it should start glowing and the iris should spin. At this stage, use Bellow. A window should appear now, with a box. Press the box and you should have a new bullet type. Charge your new bullet to the max and release at a wall, it should break.
From here, I don't have screenshots for it.
The secret boss is very tough, you will need more than atleast 60 health for your first time if you're averagely skilled. Furthermore, the skill's you have bought will all be completely useless like multishot, all except for Speed, Max Health, Splash damage and Wealth pretty much. Further furthermore, All of these become null when you activate the final boss except for your speed and max health.
Have fun with the ultimate form of Smiley, I'm gonna try to figure out if there's more secret bosses!