kill Khaos first. Don't focus on Shylphiel or else the barrier will eventually take effect, meaning Khaos will reflex, and you know how painful his magic spells are. If you focus on Khaos first, Shylphiel will reflex instead, and she's less threatening than Khaos.

And because Khaos is almost purely an elemental mage, try to equip as many skills and equipment that boost elemental defenses as you can.

Also, I think you already know this, but Khaos has an ultimate skill that takes some time to charge, so just be alert of it when it comes, especially if you're focusing on downing him first.

I saw in your video that you use Evil Swordsman. That's actually an excellent familiar to use since he has the strong magic defense to protect against Khaos. Just be aware of Shylphiel though as you're probably already aware that Evil Swordsman has poor physical defense.

The other familiar is up to you, but I personally like the Sky Dragon. Leviathan may be another great familiar to use since it has a delay skill, which is also especially helpful.

And I'm not sure if you have already captured the angel, but yeah, I would agree that then angel will definitely help out a lot here. It's good to have as many healers as possible. It's not necessary, though.

Hope I've given enough tips to help you defeat the final boss. Good luck!