Well take this Organic Life Destroying Rays of Terror... They would have to be in place for a while to actually destroy all organic life. So it's not a Death Star like thing where it fires once and destroys it instantly. It'll be more like, If left unchecked for its duration, during the rotation of said planet, the rays sit in one spot slowly dehydrates most water based creatures. If the rays are left day after day after day then their job would be complete. So, if the planets defenses are wiped out then there wouldn't be any help for them unless they have allies.