Start by killing the three soldiers around him. Once they are dead, focus on Vossler. He will be easy, as his attacks do not inflict too much damage; heal when necessary. After he is down to a quarter of his HP, he will use a move called Enrage, which basically makes him go crazy and attack much quicker. He can kill your characters very quickly this way. Summon your newly acquired Esper, Belias. Belias takes full MP to summon. You may want to teach him to a character that is not in your active party. After Belias is summoned, have the character that summoned him step back so that Vossler will target Belias. If you summoned Belias after Vossler used Enrage, Belias will kill him. Heal Belias when necessary.