This is a story based boss and cannot be missed if you are progressing naturally through the main story.
This boss is fought in the Ancient Temple after recruiting your final party member in the Sapphire Cave.
A good strategy for this boss is to bolster the party's attack with Marine's Red Orb skill and to summon Jeanne to help with healing/attacking. With Benoit, you can use his hand skills or attack physically. Karine should boost her attack with Fighting Spirit and attack. Hirado should use White Orb as much as possible to try and put the boss to sleep.
While the boss is asleep, you can use magical attacks without having to worry about waking him up (the party's "orb" skills are considered magic). Jeanne's attack seems to count as magical as well and doesn't wake him up. Use Hirado's Barrier skill to protect yourself against status effects when the boss wakes up. Take out the Eternal Warriors that he summons ASAP. If you need to use healing items or items for status effects, choose a party member to dedicate to healing. I chose Marine. Rinse and repeat until the boss is dead.