After beating the game on any difficulty you will get a password to unlock Boss Rush mode.
Unfortunately you have to enter the password every time you restart the game.
The "Boss Rush" achievement will also unlock on the easiest difficulty "Chill as Hell".


To unlock Boss Rush mode, enter the following passwords:


A4, A5, B1, C3, C4, D2, E2, E4, E5




A1, B1, B2, C3, D2, E1, E3, E4, E5




A1, A2, A5, B1, B2, C4, D5, E2, E3