STR : 7
AGI : 5
CHA : 4
INT : 3
END : 5
These were the stats that I used. And I did beat the Demon boss the first time I fought with it.
Yup. Took alot of time (approx. 35-40 minutes) of saving and re-loading a save state to basically get critical attacks (because RNG in this game is very unforgiving).
Use bind up to heal, and try to save and reload a state if the Demon inflicts massive damage onto you (you may even evade its attacks). In the end, you'll get 300 extra coins, a stat point and some rewarding dialogue.