You should equip the whole gang with Ruby Armors, Red Vests, or Red Mails before going to beat the Son of Sun.
It is quite difficult battle. You have 6 targets there: The Son of Sun, and the five “Prominencies” surrounding it. First you must not attack the core itself! It will counterattack with Flare, which does about 300 damage to everybody, and it can happen multiple times if it wants to. The core also has a nasty laser attack on its hands that it uses unprovoked, which causes 200+ damage to a character.
The thing is you can´t damage the Son of Sun directly, so you first need to attack the Prominencies surrounding it. And when you do, you’ll get attacked right back with a Flame Counterattack, which will do around 100 damage to a character. But just keep on attacking each one until you find the one that transfers damage to the core. Then wail on that one until the core uses Roulette Shuffle and disrupts your attack. Then you’ll have to repeat the pattern and find the “it” Prominency again. Keep fighting like this while staying alive and you will get the Moon Stone at last. Also If you can, i suggest stealing the Black Mail from the core, but be prepared for a Flare counterattack when you do.
