When fought alone, Brave several times with Tiz and Agnès to take out the enemy's HP all at once, but this would leave them open to counterattack if the enemy survived. Alternatively, use Default to defend against the Sky Dueler's attacks, healing with Treat or Potions if necessary, and then unleash Brave without needing to worry about BP.
When fought with the Sky Archer, conservation of BP is more important than when fighting the Sky Dueler alone. It may be wise to use Default in this case to build up BP before taking out one of the two enemy targets, then focusing on the other. If necessary, have Tiz use Treat or put the previously obtained Potions to use to heal a wounded ally.
When encountered in Lontano Villa, their weakness to Lightning can be easily exploited by Black Mages, making the fights against them much simpler.