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?