Does anyone know how to lose to the slime beast? My partner keeps soloing
By soloing, you mean you're able to skip your turn (I presume via the "Surrender" glitch- if it hasn't been patched yet, been a while I didnt try it) and proceed to not deal any damage (or protect yourself)? In such case, other than making sure your stats are very low, you'll mostly have to rely on luck and hope that the slime beast manage to hit you as often as possible (since your partner, being a "little bit" OP, will absolutely not go down in that fight, only you can end this battle as a defeat). Try to take choices that upgrade intelligence and relationships over choices that improve your combat skills.
By the way, who is your partner in the fight?
Okay so I went and redo the fight. I noticed that, if you do not try to gain strength and maximize intelligence gain at the beginning of the game, then ditch every opportunities to train, you shouldnt have any of your stats heavily modified (other than possibly strength, because you gain free points 3 time before the fight, which at best could give you 1 additional dmg per attack). But even without the 1 more dmg per attack, the best thing to do would be to protect every turn and hope rng will bless you, although it does seem impossible now that you cant literally skip your turn. The best I reached is 4/10 HP. If you really want to get the CG, maybe trying another husbando could change how likely he is going to solo the fight? Since I've only went with Ray and Max so far, perhaps you could try Kody or Alex?
Nevertheless, I wish you best of luck in your quest for the CG
EDIT: I forgot that, to do the surrender glitch, you had to be corrupted-the glitch is still available