To defeat the final boss, have a shotgun with full ammunition. Destroy the purple tentacles by shooting the spheres to get rid of the missiles it launches. Next, jump behind the red tentacles at the very back of the map. You will end up next to a wall and two of the large tentacles. You can do this on either side. The eye that shoots the beam will not be able to hit you. The only thing time that you may get hit is if the purple tentacles with the spheres come back or it does the "energy wave". Get behind it and shoot at the red tentacles. You should be able to kill the destroyer in under two minutes.