To destroy the alien you need to knock it out by punching it after it attacks. The Alien has two different attacks which can each be determined by how it moves and growls. If it is a low pitched growl and it pulls its head back use the Z button to block. He will head butt into your suit that will leave him in a daze. Then punch him out by using the B button. His other attack is a tail whip, it will be a lighter pitched growl and he will throw his head to the side. Once his tail comes around lightly tap the A button so you get enough of a boost to just go over him so you have enough time to attack when he lands. Then while its on the ground walk over to its tail, it will show you picking it up. Then with the control stick spin around and by pressing B you will release the alien in the direction it was going. You need to throw the alien out of the airlock. You need to do this 3 times.