Bombs, the best way of rescuing Phos is by bombing the earth gems. Not strong bombs, just one enough to shatter Bort and a bomb of laquer to seal Cinnabar's mercury and taking them out of the way right at the start.
Then make a rain of arrows (not explosives, don't wanna ruim the eco system of the island)
from multiple directions to make the rest of the gems scatter while Dia and Benito get down from Space Pirate Amethyst'84 ship. Send so many ships with arrow rains that even Sensei won't be able to destroy them all at once (and keep sending more!). they could also make another "professor" just to keep sensei stunned too.or even worse, cover Karen with a professor layer (since Crab is a good actress) so she can personaly ask sensei how to become nothingness or even leading him into one of the shipps to the moon
Dia and Benito get the remains of Phos. And go back to Ame's ship. While Mume and Mioga are ready to start Phos urgent repair in the ship.
Aechmea now has enough proof. That's it! It's war. The earthgems do not accept the peace agreement.