Here's the shop route I use for Normal/Hard difficulties. It is set up to make boss fights take as little time as possible. The minimum recommended upgrades to buy will be listed. Any extra money leftover can be spent at your discretion (health, earlier sword upgrades, etc)

https://docs.google.com/spreadsheets/d/1-GRcOPT2jsdNbNhTJ-I-IZFlc-YrBvjLPhL5kDAZiKU/edit?usp=sharing