Mindflayer battle needs some preparations.
Equip the "free soul" to increase your chance of escaping when bound.
Drink anti sleep and anti confusion potion before the battle starts.
Have 2 or 3 health potions and 1 magic potion if needed.
Equip stuff that reduces cooldown to get the "light beam" attack faster cuz its most effective against "Dark type"
Focus on the cyclopus first and defeat it before moving on to Mindflayer.
OR
Wait till u unlock the Onsen to get more powerful and defeat mindflayer easier.