If you're struggling with this — the final boss of the game — you can use a cheese method to kill this boss in under 10 seconds without ever being in danger.
Go to the store and buy one of the potions that increases your power. Then load up on the fire bomb daggers. In the video below, I use 18, but I had plenty more just in case. Then, chug your potion just as you get to the boss and unload the daggers into him as quickly as possible. Aaaaaand... dead.
I took a clip here if you want to see what it all looks like:
Do note that your shop does need to be upgraded enough to purchase both of those items, but that should be no problem if you are going for all of the achievements, which requires maxing out all of the shops anyway.
Shoutout to Eurydace
for telling me about this; all I did is execute it and record a clip of it!
Another tip from Eurydace: Make sure you have at least 900 orbs still saved after buying your daggers because you'll need them to upgrade the final two levels of your shop that cannot be completed until after this fight (since you need the villager for one, then a side quest will open up to obtain the materials for the maximum level).