Update on the situation: finally took her down.

Thanks for the reference, maotd and EpicFailLulz.
I took a moment to brainstorm my options based on yours and noticed I wasn't using some of my knights to their full abilities.
Then I unsealed my stack of Centies and ancient ampies to boost up Nadeshiko shrine maiden and PI Yarrow.
I shuffled the party around a bit and went with the final squad nuke strategy... and then succeeded on the first try.

For future reference:



Squad one is my evade team of choice.
I actually forgot to slap on the overheal FM on Pinkladies and Kerria and thought I'd have to redo.
I'm glad I didn't have to - the squad fulfilled its role, defeated the first enemy and chunked a bit the second one.

Squad two is quite possibly my biggest change: maxed out Nadeshiko in everything except equipment slots.
Here I made sure to have defense boosting FMs and assembled the members to boost out Nadeshiko's skill act + give her the ability to abuse other attribute weakpoints.
She was the last one standing, healing off of both her enemies and reached the final boss with full health, finally taking a part of her down with her.

Squad three is the one I actually assembled last, mainly to get any possible damage, preferably abusing the attribute advantage.
I was ready to make them fight the second enemy, hence all the slash girls, but they actually did a good job of taking Laeva's health by about a half.

Squad four was the nuker. I slapped a lure on Hydrangea, just in case Laeva gets a turn, to make sure all the abilities go off on the next one.
She did get hit once and the following turn was the last one of this stage.
Maxed out Yarrow played a good role and I was considering to add Scotchbroom (x-mas) for the chance to re-act, but it turned out unneeded.