The main thing would be getting your characters' status levels closer to their actual level.
Next would be to fix your itemization on Eiki (and probably some of your other attackers as well) because they are NOT supposed to take hits (except maybe Utsuho, since you could make an argument for the Overheating skill and Nitori for the same reason as well as Maintenance). If you're going to use Nitori and Utsuho with that in mind, it might be worth it to give them a sub class with a cheap low delay spell. They can quickly build up stacks of Overheating in theory and unleash their nuke before retreating. I say in theory since I doubt you have the items to make Nitori broken and Utsuho is much squishier than Nitori since she lacks Maintenance.
Cinderforge Sword on Eiki is fine but Regeneradors Heart and Divine Barrier need to go. Itemize your Tanks/ Healers/ Supports for SPD and defenses with SPD given more emphasis, while you itemize your attackers for damage and SPD.
I honestly think the best way to deal with the Mirror is to use hit and run tactics with Glass Cannon characters that have defense piercing attacks. You're not going to out heal the damage if your attackers are taking hits too and dealing such low amounts of damage is demoralizing, which could potentially affect your decision making as the fight drags on.
This is of course assuming you can get your SPD high enough to match it though. I haven't fought the boss in awhile and thus don't remember how fast it moves so take this with a grain of salt.
I've already spent about 500k money raising people's stat levels, so actually getting people to match their actual level really isn't an option.
Having my attackers not get hit would be ideal, but it's really not an option. Furthermore, the problem isn't that I'm not doing enough damage. It's that I'm not able to attack in the first place. He drains everyone's MP at the start, and I spend several minutes just trying to get Byakuren to buff everyone (the spd buff enhances their MP regeneration above all, and the def/mnd makes it slightly safer to switch them around) and switching healers around because he hits so damn hard. That's why Eiki (and everyone else) has divine barriers and regenerador hearts - stacking HP and defenses (fir and cld for his nukes, drk, mys, phy for his more common attacks).
I really don't have any opportunity to switch people in without risking them getting hit by a nuke. He outspeeds everyone except Aya with a 50% speed buff. His lowest delay is 7500, highest (excluding djinn storm) is 2500. Except I can't switch an attacker in after the 2500-delay nukes, since I need to switch healers in/wounded out, since no one is strong enough to withstand two consecutive nukes, even with the defensive setup I have.
His stats, from memory:
1,488,000 HP
300,000 def
200,000 mnd
900 speed (This is the same as character speed, and uses the same diminishing returns formula)
300 fir/cld/ntr/wnd
200 drk/mys/spi
150 phy
200 dth, 0 sil, I think 64 dbf, and 100 everything else (Although Kasen somehow managed to land trr, thanks to Reisen's presence).
SIL does greatly reduce his mnd, but even with sil and a 50% debuff, it's still insane. Yuuka was able to use flower shoot for about 10k damage or so. With extra attack factored in, that's about as good as the defense ignorers. And without SIL, no one is able to touch him without defense-ignoring attacks. Yuuka has 18k mag; with a 100% buff and 50% debuff, he still has almost 3x as much defense as her attack stat. Even then, he resists everything, so the damage is terrible.
What other post game bosses have you defeated?
The first 9 shadow bosses. I might be able to take out Amnesiary at this point, but I haven't tried since I first entered the post game.
