SOLARIS was relieved when the Fire Mages finally descended, including Lady Soleil's Guardians. It wasn't easy to convince the Guardians to evacuate, hence he had to force them to leave the floating island.
[I have at least an hour to keep the island afloat on my own.]
That meant he had an hour to hopefully end Aku Moonchester.
[No, it's not Aku Moonchester but the Moon God himself.]
Solaris could tell that the 'Aku Moonchester' right in front of him at the moment was possessed by the Moon God. After all, the amount of divine power that he could feel at the moment couldn't possibly belong to a human.
'Aku Moonchester' needed that amount of divine power in order to ascend that high. His mortal body wouldn't have succeeded in ascending that high in the sky otherwise. After all, Solaris made sure that the floating island would be almost impossible to be reached by the outsiders.