I only made it to wave 9. I'm not the greatest at this style of game, though knowing that there is an ending I may try to see if I can manage to beat it.
Thinking a bit more on it, giving a little more details in your tutorial might help someone pulling out the game for the first time understand things better. Along with telling players not to touch the red pentagrams, maybe including an image of the different enemies with a little description of each. I think also more clearly stating the relationship between the HP and air bars in the tutorial would be helpful. It made sense once I read your comment about it, but I didn't figure it out on my own.
And I totally hear you on the challenges of play balancing. That's one of the big things we're working on too.