Almost done with my response, though, I have something to ask going forward. 
How dark are you comfortable with an RP getting? Like, with gore and overall themes of things being horrible. It's not like most of whats ahead is crazy and bad, but, encountering Prephlin is a bigger story point and how deep we go with his domain is totally dependent on that question. He's absolutely evil, and his methods of existing are about as bad as they come. We'll encounter Medusa first methinks, but he'll come right after and I wanted to think ahead for that. 

Also, with the original guardians, here's kind of what I've been thinking. There's a neutral magic that floats towards powerful beings, this was especially true near the beginning of the universe when Earth was formed and Pahn was bound to it. Magic either enhances a consciousness or creates its own within an already existing physical thing. With that in mind, Pahn bartered with the separate elements of Earth while they were still relatively shapeless and promised to create bodies for them where they could gather their vast powers and dwell in a humanoid form similar to his under contract for their services when required. So you'd have beings that could become a magical wind or a shapeless mass of enchanted Earth and such making them nigh invulnerable. This was due to them having literally no specific point of being vulnerable, no organs and all that. They obviously became combinations of the elements with their children and less bound to the earth after procreating with other species. (Anora could maybe be like sunlight/some-form-of-shadow)

Also, just as a general rule, no being in my universe is truly invulnerable. There's always something that can cut or harm any existing consciousness. But, when it comes to the guardians we're talking about, you'd have to be strong enough to practically split entire continents with ease to harm them in the least while in their shapeless form.