Bravely Default: Secret Adventurer Boss

Reading around on the internet, I found that there's a secret passage behind the locked chest at the end of Dimension's Hasp - B10, that leads to the Adventurer and the fox that you see throughout the game.  Here, in this secret room, you can challenge them to a fight.  If you see this and go "ooh, me want", then turn encounters off so you can just run down to them without having to deal with enemies on the way.  You may also want to equip Dungeon Master for the trip, because Dimension's Hasp does contain environmental hazards and traps.

Jump break?  Jump break.  Read all about the battle after the break.

Both the Adventurer and the fox pump out physical damage like crazy.  The fox also uses status conditions (or, at the very least, Blind), and Dispel, as well as Curaga, and also gives the Adventurer more BP.  The Adventurer has 530000 health, and the fox has 50000 health.  Neither of them have a weakness.  Both of them are capable of bringing the other back into battle at full health, should you only kill one of them.  Thus, to win this battle, you'll need to kill both of them in the same turn.

My end-game setup handles this quite well.  A standard six-spell spam of my chosen -ja spell, Firaja, with Fairy's Aid up on both Vampires, kills the fox in one turn.  The Adventurer will then bring the fox back on the next turn, where I then repeated the Firaja spam to kill it again.  With the fox dying and being brought back every turn, it's just a matter of whittling down the Adventurer's health.

However, the beginning of the battle is a bit hectic.  I used Examine on both of them on the first turn, rather than My Hero -> Mimic, so there's that.  Also, this party needs to get fully set up for this fight, and the amount of damage it will be taking in the meantime is not conducive to success.

I therefore recommend leading off with Stillness.  Examine twice with the Performer/Freelancer instead of My Hero + Mimic.  The Salve-Maker/Spiritmaster will be at -1 BP on the second turn, but the Performer/Freelancer should start up their usual stuff and that'll be the last of that.  While you're still setting up, you can start staggering your Vampires' use of Free Lunch so that you'll be ready to go once you let Stillness wear off.

Continue using Stillness every other turn until you're set up with Giant's Drafts and Fairy's Aid.  Fairy's Aid can get removed by Dispel, but Double Max HP, Fairy Ward, and Free Lunch will stay.  Ignore the Blind effect, since it doesn't affect spell casting.  Later on when you've got spare turns on the Salve-Maker/Spiritmaster, you can remove it at your leisure, if you so desire.  Blind does persist out of battle, so there's that.

Depending on how long you drag out this phase of the battle, you can go straight into the build's normal rhythm as soon as Stillness wears off.  I fought the battle twice for the purpose of writing this post, and I didn't have the patience to drag out the setup phase to that extent.  I kind of suffered because of it, but I managed.

The first goal is to get the fox dead for the first time.  Until this happens, you'll take a fair bit of damage and may suffer a death.  Bring any dead party members back (my preferred method is Compounding a Potion with a Phoenix Down, this combined with Healing Lore and Holy One is guaranteed to bring them back at max health) and remember to use a Giant's Draft on them to get going again.

Depending on what's happened, you may have a slight challenge in killing the fox the second time.  After that, though, you should be good.  After a certain point, the Salve-Maker/Spiritmaster will likely not need to do anything.  You can either have them join in on the damage with Toxic Megalixirs or compounded Shadowflares, or you can just have them Default like I did.

Around this point in the battle, you should get into a rhythm and the build will begin to work just like it does everywhere else.  You get more than enough ability to recover from the things that will happen to you in the beginning of this fight.  Once you get your rhythm, do note that if the Adventurer has to bring the fox back, that's his entire action for the turn and he won't do anything else.  So basically, once you can kill the fox on every turn, you're pretty much home free.  Heck, you can even set up Auto Mode for the rest of the battle if you want.  Just make sure to renew Free Lunch and Fairy's Aid on the Vampires in the Auto Mode setup, so that they have those effects constantly.

Remember what I said up top about having to kill both of them in the same turn?  Well, here, you'll kill both of them in the same action.  If the battle had any rewards, you'd get the Sweeper bonus...

What do you get for your trouble?  The Adventurer's Mark.  What does it do?  Nothing.  All you get is bragging rights, since it's proof you defeated this boss.  Congratulations.

Post-MAGFest edit (2015-02-14): I tried some things setup-wise, but it seems that no matter what, you have to eat one turn of damage before getting into your rhythm.  Also, I ran into issues trying to use Auto mode on the battle.  Specifically, the fox would die halfway through a turn instead of at the end of it, leaving it alive at the end of every other turn, and exposing the party to more attacks.  It's also completely possible to win the battle and not kill both of them in the same action.