I find the combat very unclear, even when not that much is happening yet. How much of my energy loss is from getting hit, versus just over time? Did I get hit? Why did I get hit? And then upgrades are very unclear as well. What is my current damage? Do I keep these items? What items do I have? Most of the upgrades just do not say anything about their damage or cooldown or anything.
For most of the game, I was beating 1-3 levels before running out of energy, buying some upgrades and starting again. But at some point, suddenly I could just keep going forever and made it all the way to 25 (the end of the demo). I think it was because of the upgrade that gives energy on kill. It certainly wasn't because of the companions because they just die.