Yeah.... It is very hard. We made the mistake of building and testing all the main mechanics outside the main gameplay loop and then once I implemented the wave system and configured the values we couldn't beat it! So we spent the last 12 hours of dev time, both getting better at the game and trying to make it easier for people who haven't been playing the game for 12 hours, it was a very hard balance to find. Although we could now both beat it every time, I still think we didn't do a good enough job of scaling the waves, lessons learned for next time! :D
As for the energy we were trying to keep it mega simple, so enemies "suck the energy from you" and the glowstick gun cost energy to use, so if you run out energy the remaining monsters would get you and you've have no way to fight back. I'm not sure how well we got that message across, but again lessons for next time and thank you very much for playing!