Okay, so tips about the expansion final boss! It's not so bad... but it's also not exactly the pushover some videos make it look like.
Summary: Three phases, 17 characters. Third phase is Serious Business, second phase can practically be duo'd by Lithos and Sanae (Reisen, especially if helped by Satori, can speed it up).
And if you manage to teach Satori the move Banish the Judgment, a single party can wipe the whole thing. Poison Shield recommended for the whole fight.
Phase 1 - 2 Dragon head gate things
The top one is resistant to weapon elements but has 999 magic defense, so. It uses magic to attack. Weapon-resist-piercing or physical magic-elementalized moves are what hurts it.
The bottom is more resistant to weapon elements, but has weaknesses to the four main magic elements and 0 defenses. It uses physicals and can go down quickly.
Strategy:Make Remi's ostracon armor and get her evasion to 160. She can now dodge all physicals. You can pump her mdef up to around 130 and she's real tanky against the magic too, and can use her HP Drain moves. Cirno/Nitori/Patch/Sakuya can deal the real damage with an ice/chaser centric strategy; turn 1 Nitori can use her party camoflauge in case of a physical not targetting Remi, and Patchouli can use her Water Boost field to go last and negate the Resentment Zone field. Sakuya and Cirno's ice weapons will activate Nitori's freeze chaser, and Cirno's Ice Weapon skills are better then her magic for a few reasons. Rush the bottom gate to kill it in a few turns.
With a little luck (To take out bottom gate before it hurts the back column and before Comet is cast by upper gate) you'll take them out without issue.
Phase 2 - Fish with 2 Summoning Magic Mirrors.
The fish takes low damage while mirrors are out and can be debuffed and poisoned. Magic mirrors are resistant to things not Mystic Element and summon a variety of creatures.
-Use Sanae's ultimate snake to inflict a powerful and neverending poison on the Fish. Deals 4000 when mirrors are gone.
-Lithos' 6666 damage Last Word will deal massive damage to everything and kill everything but the fish, and Lithos can also tank great. Lithos is +++ would recommend for this phase.
-Inflict many debuffs on the fish and Reisen's Lunatic Red Eyes will deal high~extremelyhigh damage to it, speeding up the phase drastically.
Phase 3 - Sun with 4 elemental flames
The sun takes low damage while flames are out. All flames are weak to mystic, and the sun will double nuke you when no flames are alive at turn start. Sun has -very- high evasion (400!!); Youmu recommended to use her high-accuracy Paschal Slash last word or lv90 Reincarnation Slash with accuracy buffs up. You should be able to get around 300 ACC on youmu with normal stuff.
-Alice, Byakuren, Marisa, and Youmu all highly recommended. Marisa can take out flames, and a big spark is good for either wiping flames or support damage against the sun.
-Sun will use Edge of Resentment field that deals damage and stops all healing. You want to have a counter for this, and using Alice's fields can work but is risky.
-At low HP, the Sun will start using a nasty attack that deals 555 damage when not resisted. This can WRECK your party. WRECK. The amount of time spent using Mokou's commander skills to revive your party gives it time to just use it again before you get rolling. Look into Byakuren's RES buff and building up RES on people, or just trying to rush it to death after it falls below half hp.
-The first of the sun's two nukes is PHYSICAL. This means stuff like Reimu's physical Duplex will stop it if you can't use Little Legion that turn or Alice is low on HP. For that matter, Reimu is useful for healing as well, and support damage.
I haven't exactly beaten the thing yet but this is what I figured out after a few tries. Maybe I'll try again to beat it later. Methinks Satori and her innate RES and reviving skills would be very good for the final phase... but... learning those revives isn't a piece of cake.
