Arrow Keys to move. Space to shoot. Navigate using left/right keys in the guidebook. This is Boss Siege - a very, very hard and very, very frustrating game. One must beat all of the bosses in a row to win. This is just a demo, more features coming soon - note that the 'settings' and 'equip' buttons currently don't do anything, but they will.
Some sounds were taken from @theChaotic's Sound Pack.