I have both Eiki and Rumia but I haven't tried them yet. Who else is good? I'd assume Kaguya because of her defense-piercing, and I'll try to stick Discarder to him with Reisen.
Also, level 160 is... about 15 levels above my party. Ah, the overlevel is real.
If you get your library stats for HP, main attack stat, and main defense stat to the same as your level or more, 1.2 of level is good, you should be good for a while.
I used Eiki, Rumia, Kasen, Flan, Nitori, and Yugi for damage to beat the mirrior, but they really need to go through a rebalancing of the post game.
It varies too much from boss to boss, and even for the trash. After you get past the Shadow Bosses, it is actually much easier, which is counter intuitive.
If you are having trouble with the Mirrior, you can try the other one that is left on the other side of the map. I beat that one first out of the 2.
You just have to prepare for when it hits around 1/4 health, as its speed becomes insane and will murder you quickly.
As for Reisen and Hina not being fast enough, redo their stats you get from levels and put some or all of them into speed, since that is free to reallocate.
I did this for Aya, Yukari, Hijiri, and Sanae also. They became super broken that way for utility, and that is what you need going forward for all bosses.
You won't really need them for damage any more anyways.
You can probably do the first 2 deformed bosses now, or at least one of them. I forgot what the requirement for the 2nd one was...
I know the 3rd you need to clear all the Shadow Bosses first.