The Sky Archer, along with the Sky Dueler, has a much higher HP total than any enemy fought thus far. As such, expending Brave recklessly in an attempt to take them out both is a viable strategy, but it risks enemy counterattacks should they still be alive after the attack. It is safer to use Default first to guard against the enemy's attacks, building up BP so that Tiz and Agnès may attack without concern for turns spent recovering. If necessary, have Tiz use Treat or a Potion to heal.
When fought later in Lontano Villa, the party has access to Thunder, making fights against the Lightning-weak opponents much easier.