Well, there are ways to fill the slut meter outside of losing in combat. I'm pretty sure visits with the stranger increase the slut meter faster than other sexual interactions, but you don't wanna lose the MC's virginity to him, otherwise he'll pay you less every time after. And it seems you can interact with the stranger as much as you want after the unlocking point. Plus, if you do interact with the stranger, you get an interaction with the Captain in the Mercenary Camp that can be spammed. Also, if you find the Comic Book for Buddy in the Goblin Tree Forest, you can return it to him in the Mercenary Camp to give him a bj. Idk how many times you can do that interaction, but at least more than once if you don't leave the area afterwards. Just interact with the Storage Tent in the Mercenary Camp. You can start teleporting between already visited locations at the save crystals once you reach The Smuggler's (Hideout, I'm pretty sure) area, so you can fill your slut meter outside of combat and defeat enemies in combat to gain Xp. Plus, farming the first area of the Slime Dungeon, up to the first chest accessible in human form, for about 20 minutes should get you to level 7-8 and you can finish the dungeon and be ab level 10 when you face the Slime King. Also, take note that you have to level up each of the MC's forms separately. So you'll wanna farm for quite some time every time you gain a form to access it's best skills. And ALWAYS use Lyric's songs when you gain access t them. They're so OP.
But yeah, surrendering or losing to bosses is an instant game over. There are three very distinct ones in the game so far, but they're the only enemies you have to defeat on a pure slut playthrough, other than for grinding purposes. Surrendering to every other enemy in the game up to twice is fine. If you surrender to goblins three times, they take you to their orc, which is an instant game over. If you surrender/lose to the kobolds, they eventually take you to the boss arena, and you may be too low-leveled to win.