ok first step is to start the game and choose endless mode (its going to be your only option)
once you enter the game farm speed and more health
you're gonna need these three guys
once you get them you gotta drag them to their twin and for peer you have to drag it the direction the eye is looking and for drain you have to drag it the direction the small odd line is facing
here is a video to showcase it:
when you put peer and drain inside of their "twins" then they're names will turn into drain! and peer! and peer will look different. when this happens you know that they're good now so all you have left to do is go into the circle in peers eye and then use your ability (right click) and then it will show up with a question mark and you click it and then you get the cool ball gun

so now that you got the cool ball now all you have to do is get the ball at max by holding it (you'll know its at max when it starts flashing rapidly and making sounds) and then shoot it at the wall and...

it begins
basically he'll do a bunch of random attacks so thats why i said to stock up on max health and speed because its pretty hard without those upgraded a lot. anyways you gotta use your orb attack thing and shoot it at the window and if it hits ??? hard enough then it will get damaged and you just gotta repeat that until he dies
video showcasing the boss:

you will get this guy when you beat him