When facing Ethan in the bonus battle (if you took Oliva), he will start charging the black ball above his head. Equip the Surge power. When he tosses it, you can knock it back. He will catch it and throw it back three more times. After the third time, it will hit him. He will then fall to the ground and three gold colored rocks will fall from the ceiling. Use Lift to pick them up and throw them. Repeat this about four or five times to kill him. You and Ethan will then get sucked into a warp hole and get dumped in a snowstorm. Your health will drop very quickly. Walk to the top left and a strange figure will find you.