All in all, fun little minigame idea, a bit to easy to scale and win.
Some critiques and advice would be:
Increase the cost of favor each time you upgrade, very quickly I had more coins than needed.
You should cap the number of enemies that spawn simultaneously, after getting a single upgrade spamming the sword kills everything and allows the enemies to spawn until the game lags, also adding some more variety to them enemies, like maybe increasing their level or difficulty based on your warriors stats would be a beneficial feature. (adding a sword cooldown, or a stamina pool would help as well)
I would have liked to see at least 2 or 3 frames of movement for the player and enemies so they don't appear to be sliding around.