Once you reach Chapter 5 of Bravely Default 2, you'll finally be able to head to Musa and see the city that you have heard so much about, or at least, what's left of it. Now that you have all four crystals in your possession, you need to return them to their rightful resting place before they lose all their power.
However, that's easier said than done. You'll have to navigate four individual areas filled with monsters, and then take on a boss at the end of each area to return the crystal to its rightful place. Here's how to defeat each of the Crystal Room bosses, working through the rooms from left to right.
4 Wind Crystal Boss — Asura
- Weaknesses: Swords, Bows, Fire, and Wind
- Family: Demon
- Drops: Flare Hatchet
Asura is the boss of the Wind Crystal Room. It only takes half-damage from Axes, Spears, and Lightning, and is immune to Light and Dark-based attacks. It will hit you a lot in a short amount of time, so try and Default your characters when you notice Asura has been saving up its BP.
Having a Shield Master is recommended as they take the hit for anyone with critically low HP. Reraise is also a good option in case Asura wipes your party. Godspeed Strike is always a good choice, and using a Sword for it will make it deal even more damage. Make the most of its weaknesses and blast it with Fire and Wind magic.
3 Fire Crystal Boss — Catoblepas
- Weaknesses: Axes, Spears, and Lightning.
- Family: Beast
- Drops: Gungnir
Catoblepas is the boss of the Fire Crystal Room. It only takes half damage from Daggers, Bows, Staves, Fire, and Water attacks and is immune to Earth-based attacks. This boss can be a bit tricky as it'll counter to most of your attacks, so don't go all out and Brave multiple times or you'll just end up KO'ing your own characters. A Ranger's Shadow Bind will solve this problem for you as it will paralyze the boss, leaving it wide open to attacks.
As Catoblepas is weak to Axes, a Vanguard will come in handy for this fight and its Neo Cross Slash attack will do some serious damage. Focus on its weaknesses and keep on top of healing and this battle shouldn't prove too much of an issue.
2 Water Crystal Boss — Genbu
- Weaknesses: Daggers, Spears, and Water
- Family: Aquatic
- Drops: Reflect Ring
Genbu is the boss of the Water Crystal Room. It only takes half-damage from Swords, Axes, Staves, Fire, Wind, and Dark attacks. It's immune to Lightning and absorbs Earth-based attacks. Genbu will cast Reflect on itself, so be wary of getting hit with your own spells. When its HP drops below 50%, it will begin to cast Reflect on your characters instead, so make you don't accidentally heal this boss.
It will also counter your physical attacks and when you buff yourself, so don't go all out on Brave as you will likely KO your own party members. Be careful of its attacks as it will Brave up and can really smash through your defenses, so keep on top of healing. Compared to the other Crystal Room bosses, Genbu is fairly easy to defeat.
1 Earth Crystal Boss — Immortal
- Weaknesses: Axes, Staves, Earth, and Light
- Family: Undead
- Drops: Eurytos' Bow
Immortal is the boss of the Earth Crystal Room. It only takes half damage from Daggers, Bows, Fire, Water, and Lightning-based attacks and is immune to Dark-based attacks. Immortal's Sickly Spittle attack will inflict Contagion on your party members and drain their MP, but there are items you can equip to mitigate this, such as the Carbon Cap. Alternatively, you can have a Spirit Master on your team and use a Basunabringer spirit.
Don't bother using healing spells against this Undead Boss unless you are a Red Mage, as they just don't do much damage. However, Red Mage's Healaga is the exception. Of course, if you are going to attack Immortal with spells, you'll need to counter its Contagion. If you want to go down the Physical route so Contagion isn't an issue, a Vanguard with an Axe equipped is the go-to choice and Neo Cross Slash will help to deal some nice amounts of damage to this guy.