I've looked in many game walkthrus, boss faqs, and strategy guides but none of them have a really easy way of beating Son of Sun. Here's the way that I beat it, which is quite easier than anything the gamefaqs walkthrus tell you to do.
First, hit it with any level 2 magic attack. Wait for it to counter attack. MAKE SURE THE FLAME THAT DOESN'T COUNTER ATTACK IS NOT ONE OF THE TWO FLAMES NEAREST TO YOU. This only works if the flame is one of the three behind son of sun. Now, use Magus' Blackhole, and suck the two nearest flames in. Now, you're left with the three flames behind S.o.S, one of them being the target flame. Now, hit the flame. No matter how many times S.o.S does roulette shuffle, the flame will always be in the same position. It really saves a lot of time. Trust me.
PS-- in case there is someone who hasn't beaten S.o.S yet, you need to equip your party with Red mail, ruby armor, or red vests, or else you'll get pwned.