Press X to begin the fight. Select difficulty with the buttons on the right. 1 = Equip single projectile ability. 2 = Equip melee swipe ability. 3 = Equip boost ability. 4 = Equip projectile flurry ability. 5 = Equip heal ability. Space = Use the equipped ability.
Scratch may glitch out and carry death animations between games. To fix this, start a game, click the green flag, then start a new game. I dont think the final difficulty is at all fair. Have fun :D