The easiest place to get it is at the Gates of Limberry Castle.

Equip the guys with Judo Outfits and the gals with Ribbons (protects against instant death).

Take out as many Apandas as you can with the heavy hitters like Orlandu and Agrias.

Have Ramza cast Don't Move using Math Skills to immobilize both Assassins.

Continue taking out Apandas and move Ramza into spellcasting range of either Celia or Lede. If she can't move, she'll cast Ultima on anyone in range.