Make sure you have a good shotgun and full ammo. If you destroy the purple tentacles at first by shooting the spheres, you will get rid of the missiles it launches. After that, jump behind the red tentacles at the very back of the map. You will end up next to a wall and two of the big tentacles. You can do this on either side. The eye that shoots the beam will not be able to hit or damage 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 will be able to kill the destroyer in less than two minutes.