Well. You see. The spirit bomb cannot hurt people with a good will/pure soul. So killing Guardians with it is out if the question. However. The more evil you are, the more damage a spirit bomb will do. It's how Goku defeated Majin Buu(a pure embodiment of evil that can regenerate from any wound, but obviously not the spirit bomb.) So, for example, if Goku used a spirit bomb on oryx, there would be absolutely [i]no[/i] chance of survival for him.