I couldn't miss the chance to practice my drawing!
I couldn't finish 'em. Looks like this's gonna get complicated.
This swordsman of the Eternian Sky Knights uses Prey on Weak to focus on the foe with the lowest percentage of HP left -- not a very knight-like attack.
The Sky Dueler is an enemy in Bravely Default. Several of them are fought as fixed encounters on the way out of Norende Ravine, and they are later found as random encounters in Lontano Villa. They mainly attack or use their Prey on Weak ability to deal extra damage.
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.