What's a good strategy here when I want to both complete all levels but also turn her into the greatest slut to roam the planet? Even in easy mode, at slut level around 180 it's starting to get... tricky to even beat level 3. Should I just clear all levels without doing anything lewd and then sluttify her during riots (assuming there is no forced game end once all levels are cleared)?
It's mainly about not neglecting combat statistics. (The main 3 colors)
You want to have at least 1 out of the 3 (be it red, blue or yellow) be as high as you can push it. Ideally only 1. You want to unlock additional attacks (red has the super strong attack, yellow has AoE, blue has random AoE).
If you end up losing fights because of your Slut Level, then you are misunderstanding the game. Even a total slut build can win secretary. It's all a matter of maximizing Endurance and unlocking the Endure Orgasm skill, this allows you to outlast all enemies. Granted the endurance build is a slow playthrough given the amount of lewd animations compared to just eliminating them with combat without animations. The only limitation of an endurance build is limited number of combat encounter per day (one of the later passives slowly depletes maximum Energy, max energy resets in every new day).
Lastly, you want both complementary gear (Accuracy for a Red build, Strength for a yellow build, Endurance multipliers for an Endurance build), and a reasonable control/order/funds buffer.
You want to have enough funds saved in order to be able to reliably transition into a stabilized order scenario. Players that fail often overspend money for greatly diminished returns (focus on sniping strong/essential active skills, and prioritize saving gold for sniping income boosts and control/order edicts).
Easiest way to lose the game is to overspend gold on training/accessories, or insisting on very low order values throughout your entire playthrough (1-20 order).