FSIII Charles Battle

Is there any tips for beating him besides grinding my way up? All of my chars are in the level 43-44 range.

Which one is Charles again ? It's been a while since I last finished FSIII

I had to load my old save file just to make sure who he is.

Spoiler:

Thanks! I'll definitely give your strategy a shot. Grinding there is really mindless though, and it doesn't give me all that much exp...

EDIT:
I've tried out your strategy and a few other ones, and I'm still unable to beat this boss. My characters are now leveled to the 46-48 range, and my jobs are as follows: Zeto - Soul Paladin, Klaire - Soul Gladiator, Ethan - Soul Summoner, Aria - Soul Cleric. My typical attempt at killing Charles goes something like this:
(Before soul react is available)
Zeto uses Provoke then the highest damage single target skill
Klaire uses Spirit Break Second

Ethan summons Heal and Support Relic; Heal Relic is there to heal in the case of Aria having to use Lightning Aura, or if we all get slowed then he uses Relic Area Refresh; Support Relic is providing me with defense buffs. The problem with these two is that they die really fast to the single target ability that Charles uses.
I opt out of using the Ice Relic because Charles resists Ice, and he doesn't provide any meaningful utility.
Ethan also uses the mana recover soul rage skill when all my characters run out of mana

Aria is also on heal duty, but uses bless+ on the first turn.

After Charles starts using the charge skills, I end up using almost every turn available on keeping the auras up or healing/reviving teammates. My relics die REALLY fast, and they unfortunately don't come back. In theory I could hold on forever if Charles only had attack abilities. But his MP drain basically destroys any chance that I have of killing him. He uses it once, and Zeto and Klaire lose at LEAST a turn. Luckily, Ethan can still use SR. But any time after that, if I run out of MP its over. And it happens a lot. Charles' slow/haste also destroys me, because I have even less turns available to hit back at him or keep up with his other nasty abilities.

My questions are: Am I doing anything critically wrong? Am I sorely underleveled? Did I choose the wrong combination of classes to take on this boss fight?

Grinding has always been mindless after a while , especially when you grind on the later parts of the game. Just grind around the crystal so you have free exp with free heals every time.