I understand your concern. When you steer away from using proper nouns (which you should steer away from doing), it becomes tricky to find a title that isn't already used. A quick search of books on Amazon turned up nothing with a plural "Supernaturals" in the title, so you seem to be okay from a trademark/copyright perspective. You can absolutely keep the title if you like the title.
Now of you want to change the title, I can see a few easy fixes. You could flip the subtitle and title and subtitle: The Earth Titan: The Supernaturals Book 1. You could combine the title and subtitle: Supernaturals and the Earth Titan. ...or The Supernaturals Against the Earth Titan. You could add a strong verb: Awaken the Supernaturals or The Supernaturals Dance. I don't know what they actually do in the story. Any of these tweaks would remove your story a step from the Supernatural fandom which is really all anybody could ask.