Boss: Chaos
HP: 20,000(!)
Attacks: Blaze, Tsunami, Earthquake, Cyclone, All lv.3 elemental spells, Haste, Slowra, Curaja, Flare
This guy. Whilst most final bosses can be pretty easy this guy is a beast. I had a Knight, Ninja, Black Wizard & White Wizard. Equip all your characters with protect rings to stop Earthquake ending your attempt and I equipped all 3 of my ribbons to everyone but the Knight as he take magic hits like a boss anyway. Make sure to have Excaliber and Masemune (but that's not essential).
First turn: Get your main Physical attacker, so my Knight with Excaliber (just one of them, don't have everyone attacking) to use Giants Glove on themselves. Have your Ninja, or other secondary attacker, use an item called White Robe to cast Invisra on the whole party. White Mage use Healra/ga and BM cast Haste on your primary attacker.
Your main guy will be constantly attacking and WM constantly healing. Use the White robe a couple more times and then have your secondary attacker either throwing items around like a mad man or in my case casting Temper to up your attack. Your BM will constantly cast temper as well (It stacks with every cast.
After a few turn your main attacker with Excaliber will be hitting for around 2.5k and with some luck that his attacks don't kill or injure your main healer and attacker you should take him down before he even tries to heal himself. If your secondary attacker or BM dies it might not be worth the revive as a turn of none healing could spell doom.
Just a note that while I managed to beat Chaos at Lv.41 with a Knight, Ninja, BM & WM half of it is pure luck on his attack pattern. Mostly he attacked with his single hit physical but a few multi target spells in a row will kill you. Have patience and Good Luck!