The Adventurer is an optional boss in Bravely Default and is the hardest adversary, that is not a Nemesis. As in Final Fantasy: The 4 Heroes of Light, the Adventurer is fought alongside the Faithful Fox, known in-game as Comrade.
The Adventurer will start by using regular attacks and Mass Slash, which deals severe physical damage to the entire party but costs 1 BP. However, to assist with BP, the Comrade will use Adventurer's Friend which gives the Adventurer 3 BP and costs the Comrade nothing to use. This becomes very dangerous as the Adventurer will start pairing attacks with Brave, usually either using Mass Slash twice or Meteor twice. With dwindling HP, the Adventurer will start getting desperate and Brave into negative BP more often, but will rarely Brave more than once.
Meanwhile, the Comrade will assist the Adventurer with Blind and Dispel, which are always group-cast, delaying the party's physical attacks and forcing them to re-apply buffs. In addition to Adventurer's Friend, the Adventurer offers direct support through the use of Curaga once HP becomes low. If the Comrade is defeated, the Adventurer will revive him on the next turn with Call Comrade. If the Adventurer is defeated and the Comrade is still alive, he will use Arise, fully restoring the Adventurer's HP.
Because the Comrade has the Protect Ally passive ability, strong group-cast attacks work best against the Adventurer. A Vampire's genome abilities, especially Firestorm or Firaja, Blizzaja, Thundaja or Aeroja, work well. They are also helpful for reducing the Adventurer's P.Atk and M.Atk through Absorb Stats and Curse. A Templar is also useful for Rampart which will minimize the damage of Mass Slash. Similarly having a high M.Def will reduce the damage of Meteor, which can easily deal 9999 damage if it hits one character three times.
Once the Adventurer is almost dead it is vital the Comrade dies on the same turn, or it will be basically game over. Therefore if group-cast attacks will leave the Comrade alive, use some single-target attacks to reduce the Comrade's HP before taking them both down.