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.