I beat this pretty easily. A few issues.
When you transition, you can see your character in a black screen for a moment. Instead, add a fadein on the transition target and pop your character in before the fadein there instead of fading in first then transitioning.
Spikes cannot kill you, so you can just be at 1 hp and run all the way to the final save room, then teleport back and farm the first floor with the gear you've collected.
The vending machines make it very easy to accidentally buy an item.
It is, in theory, possible to beat the game in two days (of 7) if you buy the right items the first time around.
The pistol upgrade has the wrong skill attached.
The first floor is the best place to farm for levels because it is easy to kill the enemies and with enough defense, the enemies can't hurt you. Especially if you have the thorn armor or the demon gear.
Didn't see any use at all for collecting all four cards, not sure if there was anything regarding that or if they were actually useless.
The shotgun is honestly pretty weak for how late you get it.
The healing items seem best used outside of battle and only when you are farming. Otherwise, it is better to stay at 1 hp and keep thorn on.
One of the enemies has an absurdly high counter rate, making thorn armor almost necessary as a way to kill it, but you only get thorn armor as a random drop or in a later chest.
You can die in the tutorial-if you end the tutorial with 0 hp, it games over. Maybe have an event that checks if the player's hp is 0 and heal them if it is. Or have an invincibility state, revive the player AND full heal after the battle, THEN remove the invincibility state.