Is Deamon King fight posible to win ? and how ?
Luck and Mesmerize. Even when equipping yourself with the best items and not cheesing with the super spell, you need luck to survive past the succubi.
If you do, as soon as they're defeated, Mesmerize Gustav, buff your party twice, debuff him twice, and then attack while buffing yourself, after that throw Mesmerize until the buffs run out.
Use these turns to recover MP with items while using Mesmerize only when he wakes up or you think he's gonna wake up (if short on items, prioritize Sana's MP, or if no items, Guard and Mesmerize for however long you need). When you think you're good to go, repeat the initial process of Mesmerize and buff-debuff until you win.
I think theres a pure ending where Sana doesnt fuck with anyone or almost no one
Yes, in that one you only get Europa being jelly. I'd consider it a variation too, albeit one where you have to protect her and choose to believe the best
/ be wilfully ignorant for the entire game.