I'm not good at the game but, how can I beat the mushroom girl
Go to the start location and hit the totem to summon 🍄 monsters and kill as much as you can to obtain lot of G (money) and sell all the mushrooms you got to Cathela (Caterpillar girl). Then buy all the 🍄 soup you can (it recovers health 100%).
My last advice is to put the soup on the 0th item slot and use the special button to eat while you are battling!
I hope this helps!