So two things:
1. You want new players to start off with quick wins so that they can appreciate and get hooked by the simple and addicting game loop you have going on in this neat game.
Solutions:
- Lowering the health for the first few skellys coming in. they increase in health as they continue spawning
- Give players a lot of power and fire rate then taking it away as a promise of what's to come
2. With some push back, screen shake, or briefly turning the red for a fraction of a second, the feedback when hitting the enemies would make this addicting game play loop ever better.
Keep it up, this was really cool!