Iku Nagae Boss Fight:
This one is a little tricky but if you can memorize the attack patterns, you should be able to beat Iku in a few tries. Iku will cast lightning shock which will enable the user to receive death status at the end of the turn. This is by far the most devastating and annoying attack.
Have Reimu cast the barrier first thing. Have Youmu and Sakuya as melee fighters with Byakuren enchanting the weapon with earth element. You will likely need to abuse Sakuya's Private Square if Iku didn't cast the lightning shock attack and you end up mis-timing the cast of Reimu's barrier (has a 2 turn cooldown). Use Sakuya's private square before turn 4 or so or Iku will start regenerating at a rate of 666 HP per turn. If Youmu has a high critical rate, she can do massive damage (3000-7000) per turn with buffs with her strongest attack.
If Byakuren can't cast any other buff magic, use Dark Sphere on Iku. Have Sanae to buff power up spells. Defense spells isn't needed.
Recommended party setup: Reimu, Sanae, Youmu, Sakuya, Byakuren, Aya(commander).