Use WASD or Arrow Keys to move. Dodge anything that is red. You can't attack, you have survive until the end without losing all your hp. If you are really struggling, then click z to get infinite health. (Yes, it is possible to no hit the whole boss battle)
I enjoyed the original project so I decided to make it harder. I added a sprite to the red cube, and added cool features such as health, red walls, a winning/losing screen and a cannon at the top. I also fixed the w button because for some reason in the original code it wasn't included in the player's script. So now can use WASD. (Yes, I know, the cannon design is really cringe)