for the other boss stages before Formalhaut 3, you need to have broken the seals. this will give you flags.
1-4, fly me to the moon, lauch enemy upwards as high as you can
2-4 defeat the enemy on turn 5 or after turn 5
3-4 deafeat both enemies at the same time. careful, the left enemy will resist the first death so wittle down both hp as small as possible, kill off the left enemy for the first time, then burst to kill bothat the same time.
once you get all three flags, start Formalhaut -3, click your items and you will see your flags where you normally use potions. activate all 3 flags for instant win.