Hey, thanks so much for trying it and for the awesome feedback! Yeah, I think it definitely needs to be more obvious when you're dealing damage to certain enemies. Also it's interesting that the energy source one-shot you as that shouldn't happen haha... was there an enemy lurking in the same room? I may need to prevent enemies from spawning in that energy source room.
Regarding checkpoints, I'll see how that feels in the final game before making a decision on it - for the demo level I'm spawning in all enemy types as I wanted people to experience them, but in the actual game a lot of those stronger enemies won't appear until higher levels. So hopefully the difficulty will ramp up slowly. There will also be powerups and upgrades to help you. That said, if it still feels too unfair at that point then yeah I might consider some kind of checkpoint system :)
Thanks again!