Whenever you are about to fight a boss make sure Rose is in your party. You can tell your about to fight a boss if u find a save point deep into a dungeon. Have your SP to the maximum for each party member and make sure that each member can transform into a Dragoon. Once the battle starts use Rose's special and cast Astrol Drain as many times as you can. I've been able to do up to 400 points of damage with it on the first disc. After the special wears off keep using normal attacks and additions so that your spirit gets to its maximum fast enough to use another special. Also, if you have the power wrist equipped, use the special of that person so that a normal Dragoon attack can deal near 200 points of damage. Make sure you have plenty of Heeling Potions and a good amount of Mind Purifiers incase the boss can cast status effecting magic (like the beast in the Hellana Prison dungeon.) Make sure you save often because bosses often come in pairs of two.