If you have trouble either understanding the step-by-step or how to fight the boss, there will be a link to a youtube video at the bottom of this guide where I show you how to do it, as well as how to beat the boss!

You will have to do the following:

1. Clear the first room.
   
   
2. Go to the door at bottom of the screen and get as far down as you can without entering the next room. Once you've done that you will be able to get behind the walls on both the left and right side. The best way to do it is to tap down fast and repeatedly, and move either left or right to see if you're down far enough to get behind the wall!
   
   
3. Enter 10 rooms by going to either to the bottom left or right corner whilst still being behind the wall. You can leave a room as soon as you've entered it, since you aren't locked behind any doors! If you have trouble with remembering how many rooms you've entered, the last room before the boss spawns a bunch of blue shells, so keep a look out for them!
   
   
4. Once you're in your 10th room, (excluding the starting room) just roughly center yourself at the bottom middle of your screen and dodge roll down. Once you've done that you've entered the boss room!
   
   
5. Beat the Boss and get the achievement!

If you have any issues that I've missed in either the guide or the video, leave a comment and I'll try to help you!

Good luck!