Really, if you aren't overleveled and come prepared, it's a pretty easy fight. Make sure to have people survive Forbidden Fruit (having Reisen on the front line with Wavelength of Insanity soaks up enough damage for your front line to not die) and enough of your nukers survive until she uses Great Catastrophe, then just murder her. Pretty simple really.
I level down to below challenge level when I face these bosses to get more prizes. So far Youmu can survive everything. Except that level 128 boss I accidentally ran into with crazy regen.
Using Aya in that kind of way is pretty cheesy honestly. Aya isn't that tanky, she has terrible mind albeit she has good HP. You're better off putting her in slot 3 imo.
Not too sure if having terrible mind is that much of an issue. Youmu, for example, has no trouble being a slot 1 tank and never really dies. But then, I did give her 20 training manuals and other power ups and at level 65, she has 70 levels to stats, 20 levels to affinity from library.
I also specifically not give Youmu MP, because that is counter productive to using Meikyo Shisui.
Just build Aya in the HP department and you should be fine putting her in slot 1 or 2. The evasion tanking is a nice bonus, but she can survive a big hits from bosses.
Aya's cheesy due to her evasion, huh? I wonder if Alice would be considered more so due to her Doll Guard skill, which pumps up her evasion to be better than Aya's and that's before you get a Tome to give her the EVA Boost skill.
She is cheesy, because of her skill set and speed. She can stay in any battle permanently and speed buff your team automatically. With Healer, she heals and speed buffs your team's speed really fast.
Have her in slot 1 or 2 with pure HP build. Give her 1 Meteor Drive, Arch, Lambda Drive or Star of Elendi, depending on the situation.
You use Guidance on herself. Then, just do normal attacks or put a second Guidance on herself. She should have taken 3 to 4 actions before any character gets their first action. This gives them a nice 24% speed bonus and only goes up from there.
She doesn't work too well with Hina though. Since she will passively buff Hina's speed and you want it in the negatives instead.