I was in the process of doing the same, But given once the new version comes out and id most likely have to do it again and there most likely isn't like an in game change by achieving it, i kinda gave up on the grind.
Chances are big in the future our pink haired senior in sluttery will give us a scroll which explains mouths can also be used for pleasing others, alas at this moment only the rare chance of kissing increases it.
Aaawww, that's a shame
I guess I'll just make a backup of the save, finish the game here and then wait for a future update where slut senior gives us the magical ninja scroll containing the instructions on how to give head without accidentally breaking your neck or something and then proceed with the 100% slut run
That way when the update containing the updated sex skill tree, I can simply slap my save file there and not need to stand in a corner between two guards letting them passively double team Luna while I play Blue Archive or watch youtube
It's a shame though, I would've loved to finish the game at "full corruption" like I usually do in these types of games for the first playthrough
It is both fun and amusing if you think about it in world canon wise
Ninja person: "Sir, the apprentice has returned"
Ninja master: "Was she caught?"
Ninja person: "Yes sir, it appears that the entire army has seen her and knows her"
Ninja master: *sigh* "So she failed... Get someo-"
Ninja person: "Actually she succeeded, and without killing anybody except for the target as well"
Ninja master: "...what?"
Ninja person: "In fact, now we have the entire army as allies"
Ninja master: "What?"
Ninja person: "And they're having a 60 men gang bang right now in the storehouse at the village's outskirts"
Ninja master: "W H A T ?"