Yeah that's me wrecking the game with Tenshi. I have no complaints with her offense, relying on Iku is fine. Iku herself is a superb support character, and I feel that synergy skills should be strong like that, since they rely on very specific characters. Granted, my entire party and playstyle has been designed around Iku + Tenshi, since my intent from the very beginning was "Tenshi is my favorite Touhou character, I want to build a party around her". I think Tenshi is in a great spot overall despite the bias though.
Regarding her DEF/MND, it is true that in Plus Disk, it's pretty lacking in value (Tenshi was extremely durable in the base game though). I suspect that 3peso was aware of this when he added Seven Celestial Peaches to Tenshi, and buffed it in a later patch (it was originally only 4 levels, which was really weird because the skill name was Seven). Tenshi is also blessed with excellent affinities all around, so she excels in all forms of defense, outside of MYS and DRK attacks for her weakness elements. Even so, her weakness is only 75 (so it caps at 150 at library 100, 182 with Affinity Boost), and those are good elements to be weak to because Machine God Lucifer adds 99 affinity to them. Even if this were not the case, Regalia adds so much affinity that a character's base affinity is almost irrelevant. +172 to all elements on top of a +600% HP boost is absurdly good, especially with how much easier it is to create compared to all the other top level equipment items in the game.
Regardless of all the ways to ignore this problem though, there are still sometimes cases where an attack has a low ATK/MAG modifier, but a high damage modifier, and Tenshi can still tank those easy.
Regarding the debuffs on World Creation Press and Violent Motherland, I feel that having both an ATK and MAG debuff on World Creation Press may be a little too strong, as it is already a devastating NTR attack. Similarly, Violent Motherland is a powerful attack for dealing with bosses that consist of multiple targets, and the SPD debuff is quite strong considering the damage being inflicted. That it debuffs MAG as well is another small bonus. I do see where the problem comes in with Tenshi as a tank character though. When she's in that role, there is nothing she can do to contribute in terms of support besides switching, dispelling buffs, and subclass effects. The debuffs on her spell cards grant her additional options to support with, but in the case of Violent Motherland, it is too expensive and has too much delay to be useful for the purpose of debuffing SPD. But any adjustments that can be made would have a more pronounced effect on Tenshi in her attacker role, so it is difficult to address this in a balanced and fair manner. For now, I would not say anything about it. Instead, I want to focus on getting Former Murakumo's Owner for support. If it was to activate regardless of the presence of a status condition, then that helps alleviate the need for healing. With the passive healing effect active, in situations where Tenshi would be switching a healer in, she could instead be using a debuffing attack. It would also increase the usefulness of switching with Tenshi, as it would speed up her healing ability.
Keystones of Spirit, conceptually, is likely based on how the Sword of Hisou is supposed to have the ability to change form based on a target's weaknesses. As far as how it actually plays out in gameplay, Tenshi's enormous ATK potential allows her to make excellent use of subclass spells that scale off ATK. The damage potential from striking a boss' weakness is enormous, and in many scenarios Tenshi's best offensive option is indeed using a subclass spell instead of her own spell cards, even with the presence of Murakumo's Blessing. Ignoring the 1-character subclasses, Warrior's Explosive Flame Sword, Monk's Puncturing Thrust, and Ninja's Assassination Sword are cases where the spell card is strong enough to exceed Tenshi's personal spell cards backed by Murakumo's Blessing, provided that they are fully taking advantage of the target's weakness (I.E. Divine Falchion, Satori and/or Akyuu). Due to Tenshi's high DEF, with some excellent ATK + DEF boosting equipment she can even make good use of Guardian's Shield Bash, which will actually offer similar damage output to the aforementioned spell cards. The areas that Tenshi lacks are in CLD (Samidare Slash is too weak to surpass World Creation Press or Sword of Hisou unless the target resists NTR/SPI and is not weak to anything besides CLD, no such boss exists that I can think of) and MYS, which is something that Iku luckily covers just fine. The only real problem with this is that Tenshi needs to switch subclasses frequently to make full use of Keystones of Spirit, but this is necessary for any character that is trying to target a weakness they don't hold a spell card for.
Swordmaster as a subclass is kind of bad, so it's fine if Tenshi can't use it. The only reason for her to use it is for Samidare Slash, which is regrettably bad. The only other useful spell card it offers, Explosive Flame Sword, is done better by Warrior. Swordmaster could use some buffs as it currently is.
For Enduring Celestial, I think that it's fine as it currently is because of Courageous Sword. At the start of the fight, that is a 13.2% increase to ATK. Furthermore, with the Herbalist spell Placebo Effect (which is one of the best options for maintaining high buffs on a single character, due to its low delay), after any kind of DEF/MND buff (such as from Enhancer heals or Sanae's Miracle Fruit), Tenshi has enough DEF/MND for Placebo Effect to maintain an 86% buff on her with relative ease. The overall result of this is that Enduring Celestial makes Courageous Sword easier to use, whether for mobbing or bosses. Back in the main game where DEF/MND actually mattered, Enduring Celestial also made State of Enlightenment unnecessary to use, because Keystone Formation helped to maintain a high enough DEF/MND buff as to make it unnecessary to use State of Enlightenment to make Tenshi durable enough to tank. Because of that, it made it easier for Tenshi to focus on switching instead of wasting her ATB on buffing herself, increasing her productivity as a tank.
So overall, I feel that Enduring Celestial is actually really useful to have. The main problem with it now is simply that DEF/MND are not valuable at the moment, so it doesn't demonstrate how useful it is when combined with Keystone Formation.
Some of the things that were posted did make me realize some flaws with Tenshi that may be a good idea to address.
-Because of the low effect of DEF/MND in Plus Disk, I would like to see an additional effect to Keystone Formation, such as reducing the delay on switching.
-I would like to see the delay on Freedom from Worldly Thoughts reduced, and the demerits easier to acquire.
The latter is because PAR is useful for Iku to remove (20% to herself in all stats as well as Tenshi), and I want an effect on Hina that lets her receive status effects from other frontliners, so that she can activate Two-Way Curse more easily (HVY would activate Spinning More Than Usual). Presently, only Tenshi and Remilia can afflict themselves with status effects, IIRC. I think it would be good to make it easier for the player to intentionally inflict status effects on characters to take advantage of skills that rely on a party member having one.
Speaking of Hina, this made me think of some other ideas for her.
-Regarding the skill Spinning More Than Usual, I would like to see an effect that causes Hina's ATB to become 10000 upon receiving a PAR effect. I would also like to see PSN added to the usual effect.
Initially I was thinking that I wanted Hina to have an HP recovery effect upon getting a turn when PSN'd, but I think that's overly complicated when there's already a perfectly good effect for it to be used with.
Re:Passives; Keystone Formation only even activates in a tanking setup and she's useless for that in Plus, so changing it doesn't matter; it's effect is desirable before Plus content as-is and useless after no matter what.
Wanted to address this more specifically, Tenshi's not so much useless as a tank as way,
way better as an attacker. Iku is one of the best single-target buffers in the game, with low delays, high SPD, low MP costs, and even higher DEF/MND than Tenshi with Hisouten Guard, with similar levels of HP (Tenshi w/ Seven Celestial Peaches has about 5.5% more HP). Her MND is such that even with minimal investment in it, she's taking 0 damage from some of the 30f mobs that use VOI attacks. She also packs the strongest DEF debuff in the game that also comes with a decent PAR effect, as well as a strong MT DEF debuff. Iku has all of these things going for her as a support character, while Tenshi has virtually nothing useful going for her. While she can remove buffs, this is something she can do even better as an attacker, since she's spending every turn attacking that way. While she can tank, she isn't doing much besides switching, and she has no passive ability to increase the damage of the frontline. She simply does not excel in any way unique to herself as a tank. This is one reason I feel that lifting the status/debuff requirement on Murakumo's Former Owner would be a good idea, as 5% HP regeneration per turn can be fairly useful. Keystone Formation probably does need to be buffed in some way as well.
One idea I have for it is to give it a counter effect on Tenshi that reduces the damage done to the whole party. The counter would be increased by performing a switch and decreased whenever the boss damages a character. This seems excessive though, especially since Tenshi already has Keystones of Spirit counters. I'll stick to just asking for Keystone Formation to be improved.