Thanks for playing this game!
I see you had trouble with the "boss fight" that takes place at the end of the game.
My tip is to constantly use "Heroes' Bane" whenever possible. It greatly increases your chance of survival.
To be honest, the blank characters (blank slates) were not actually supposed to be there, but I did not get to check that part so it also improves survival rate.
The boss' health is 10000, and constantly using "Heroes' Bane" and "Double Attack" (In 'Special' menu) will get you the win.
It might take a couple of times to complete, though.
For the "Shopkeeper's Coupon" that you got, it's a way to be able to skip the more-or-less 5-minute thing at a 10% chance.
In easy mode, you can pretty much use the save system (Talk to the skeleton to get it) to exploit the coupon.
For getting the coupon in Hard mode (irrelevant if you already finished the game in easy because it's pretty much the same thing for now), the coupon is between the three pillars on the right side of the door/gate that opens up to the 'castle'. You can 'go under' the middle pillar to get it.
Thanks again for playing!