Vossler is a weak boss and should be easy to take out. He will be accompanied by three Imperial Swordsmen who are of level 17–19, and have a 50% chance of having Haste. They have 845 HP and are immune against Sleep, Reverse, and Lure.
Fran will be afflicted with Berserk, and can dispose of the sidekicks with ease. If Basch is in the party, Vossler will only target him; without Basch, he will attack the party at random. When Vossler senses magick being cast, he will start casting Reflect. His initial Shell and Haste protection will wear off as the battle continues.
When Vossler's HP is depleted by 40%, he will start using Enrage, which enables him to attack instantly. This will help him build a combo of up to 12 hits and can be dangerous.
When the battle ends Vossler will apologize to Basch, and ask him to continue his duty, protecting Ashe.
When Vossler starts comboing his attacks at low HP it is worth using a Quickening chain to end the fight.
The party could also summon the newly acquired Belias whose Painflare inflicts considerable damage, particularly to the Imperial Swordsmen. Hellfire could finish Vossler off, depending on how quickly the swordsmen fall. Vossler switches his target to Belias almost immediately Belias is summoned, and, considering the MP spent to summon Belias, Potions are recommended for healing him.
If the party has a sufficient number of Quickenings they can immediately aim all Quickenings at Vossler, and with a sufficient chain, can wipe him out. His sidekicks will not be a problem: as soon as Vossler is defeated, the battle will be over.