You mean Convoluvus? That one Convoluvus I didn't get from her reissue gacha ;-;
Edit: It could be doable with Spade-chan if that guts wall wasn't broken
edit2: Okay it seems this boss really has a tons of guts and not something guts reseting when above 1 HP...
I'm using Balsam+Spade-chan instead of Convoluvus. It works well but I need more power to reach the guts wall earlier. right now I could do this but I always reach the 15 turns limit.
Balsam is still lvl100 with only max high ATK ampies. I can raise her HP and Def and push her to lvl 100. And max her 3rd affection and give her an Oath ring.
Edit2:
Finally oathed Balsam and Cardamine:
I don't have enough rings for Pink Miko and Kerria (and for RRose but I don't have enough heart too). I can't wait the next and next next memory shop refill.
And I did lvl100 Hard on the test run. Very hard and very close. But doable with an insane amout of luck on Balsam re-act.
Edit3: Lvl Hard : Done.
I used the magical combo of Balsam/Cardamine/Pink Miko/Spade-chan/Mistletoe. Balsam did everything. And as expected, the combo Spade-chan+Cardamine make Cardamine immortal (at least on ennemies without instant death attacks). Spade-chan+Balsam is pretty good too. But Balsam needs some heavy skill-act support. Thanks to lady luck who blessed me with Spade-chan on the last gacha. I would not me able to do that stage without her.
Big thanks to Raine who reminds me how good Balsam can be. I left her a bit and it was a mistake.