Wasn't worth it.
The best strat I've found is Fire+Ice stone. You can instead of ice use "imagined X" stone as it gives the required Godly Speed skill
Skill order:
Extinguish - important buff get MP as you spend TP -"Free action"
Fire Wrap - only for hard bosses otherwise not worth the cast time - "Free Action"
Ice field - only for hard bosses otherwise not worth the cast time -"Free action"
Godly speed - important to use as last buff skill - two actions per round doubles mana cost "Free action"
Spiritual Liberation - spend all MP get TP based on MP spent
Exorcism skill - Spend all TP and we are getting all MP back due to earlier buff
All of that in one turn, do keep in mind that Extinguish buff is 3 turn duration 4 turn CD so on 4th turn you should not use the spend all mana skills - either cast something small or defend.
Get stones with as much magic attack as you can. The formula is like (MATK - MDEF)*coefficient so if you don't have enough you ain't dealing shit and if you have enough you can damage the "physical" opponents with magic as they just have higher base MDEF.
For Kana - she is easier to equip as certain cards will always spawn with certain stat - so strength will always have ATK and Hermit will always have defense. So than what you can do is : Temperance +Strength + Strength for card with massive atk. For defense i used World+Fool+Hermit. Cards do have their levels that are hidden so check check if you get stronger ones and keep deleting the weak as there is inventory cap! (100).
Skill wise she can use taunt+sunder for free stun (MC needs to stun the other). Alternatively if enemy is already charging if you use taunt they will go into "super charge". If you hit them with Kana's sunder (you need to do it on same turn as taunt!) she will do massive damage.
EDIT: You can also trade her your enhanced crowbar and enhance the other again.
True Ending:
So you need to play with the spirit girl 2x on each map : abandoned house, church , hospital. Those areas need to be leveled! 10 is for first event and the second maybe 15? i had them at 18-19. You also need to complete the quests for citizens check the guide in OP and another here
https://f95zone.to/threads/rebel-geister-jaeger.117016/post-9552687 I'm only mentioning what those guides didn't explain well.
Examine hidden room in mansion (mask on wall where you find map, after you kill "eye" boss and enter the house, north wall), have high love points with Kana and go on dates with her so the required events play out. (after examining the room you need to go on date 2 or 3 times assuming you have the other conditions
She tells you truth about herself and than MC needs time to rethink it how to solve it all ). Than kill the boss in mansion (you get save prompt before it so you will know). Mansion area level goes down as other areas levels go up, you can get it down to 0.