Elements: Soul of Fire

After the ending, you can free roam the world. This Hidden boss is located in the first town, go to bed in Leonardo's room to encounter him.

He has three phases and mostly does physical and darkness attacks but also has a Banish move that can potentially eliminate one of your teammates from battle (you are unable to revive them) so be careful. With a good party setup and around level 85, you should be able to beat him.

I personally used:

Leah equipped with her best weapon, Hobo Lily, Corset and Ogre Badge.

Maximus with his best weapon, the 2xSTR Shield and that lyre that overcharges his charge move (from the other superboss)

Pica for support using Star Cross to weaken the dude and Giant Wall to buff our defenses.

Leo also had a supporting role by using Stat Raiser and Life Guard items along with Jubilee party heal pots.

Good luck!

To obtain this awesome combo visit the sister and brother duo at their home in Hobo city after they leave your party due to special circumstances.

This achievement is missable.

You need to bring the bow of sagitarious, the scales of libra and the jug of aquarius to a dude at the northernmost area at Castle in the Sky.

The bow is a rare drop from centaur-like monsters.

The jug is in a locked vault at the underwater castel and its missable if Neptune dies before you get this as you need him to breathe underwater.

The scales of libra can be bought from the Thief at Ikory after you defeat him.

After killing Emperor Fiyara for good, go back to Glymac castle with Jessica and dance with her at the dancehall located in the west wing of the caste, in the library's basement.