I was 19 when I took him down on hard mode.

Turns 1-3: Default/group heal.
Turn 4: Unleash phsycial barrage, and lighting (I had the lighting empowering rod equiped.)
Turn 5: Normal attacks/Heal.
Turn 6: Default
Turn 7: Default/Heal
Turn 8: Default to defend from Acid Breath and not die.

Unless you are unlucky and he clocks your healer turn 7, your group should survive the Acid Breath, and it's just repeating that pattern. Having a couple people that can heal helps.

Just know his third attack after Gluttony is always the Acid Breath, and he's not to bad.

And the Merchant and Jackel weren't so bad for me, just make sure to have a dedicated healer, a second person to back up the healing, and kill the Merc as fast as possible. Make sure your equipment is up to date as well, I smurfed that up the first time and my Knight was hitting him for virtually no damage.

I just cleared out the Wind Shrine, and I must say that while I'm loving this game, there are some problems.

Mostly I find both Tiz and Agnes to be really bland characters. They aren't bad, just eh. And I like Ragna, but he hits that 'I like the ladies' joke way to much, it get's old pretty fast. So far the characterization any all the characters isn't really all that great, though not bad by any means.

Enjoying the smurf out of the game though, dont' get me wrong.