Or... or it can just be a dreamfight. Like... you go to sleep next to Perpetua and the remnants of the tainted core seep into your dream and try to catch you in a battle.
I don't think it has to be too complicated a setup to work.
Saying "even the basic blues" doesn't really reverberate imo...
Basic Blue slime has a special CG but Shizu doesn't after all.
Most of the game has no scene CGs, and it's not expected that they would. I think it's a welcome surprise whenever we do see one. Original Perpetua didn't either.
Hell Kotone, who's the highest effort girl in the game has no scene CG either...
Probably for good reason though... the CGs honestly make no sense logically.
Take the basic blue slime CG for instance. Look at that confident expression. Look at that position.
That's a CG saying the girl's leading. In game though? You as the MC can choose to insert, and fuck her into a messy pool. The CG isn't reflective of that at all.
That's the case with most girls in the game.
Think about Shizu. She's half sub right? Imagine, MC inserting and then getting a cowgirl CG for it... That'd make no sense based on her personality and how the fight goes if the MC initiates... but it'd be the pose that makes the most sense for her, because most players first encounter her with her riding your dick as you wake up from sleep.
What about Bubble Slime's CG? She has skills that imply she's being assertive, but her lazyness, and the general likelihood she'll ever reach that stance is like... 0. It's more likely that the MC will choose to fuck her after he's done fighting her so what would her CG be?
The only battleable girls so far where the CGs would be canonical to any action you take in game are full subs like Alraune and Matango, or the explicitly overwhelming girls like Mizuko. The alraune, matango, nara etc type girls are easy to explain. The MC is always the one to initiate regardless, so giving them their doggystyle CG would always work.
Overwhelming girls like Mizuko, and Venefica have writing where even if you rush to insert they force you into their preferred position anyway (Reverse Cowgirl for Mizuko, and Cowgirl/Amazon for Ven.)
All other girls are a mixed bag, and commissioning scene CG for them would mess with their battle feel.
Hell, blue slime feels weird already having that CG and that's undeniable imo.