It seems that you can win with only fire mages. I haven't used anything but fire mages (other than just trying some hunters) so far and haven't had any problems. I fully upgraded everything I need a while ago and I'm just trying new things. The biggest challenge is getting enough mages down and upgraded in the first few seconds so I don't get overrun. I tried the same thing with hunters but I seem to always get overrun in those first few seconds. If I do the same with mages, it's just a matter of waiting for the level to end.
I rated this a 4 because while there are plenty of flaws, there are a lot of good things about this game and it looks great. You seem to really care about how the game is received and have been making fixes. I hope you make more games in the future.
Perhaps up the login bonus. I get three times as much money and experience every tick than I do logging in. Also, holiday achievements would be fun. And maybe an avoidance kind of mini-game. Something as simple as dodging things coming from above or even some type of flying game to see how far you can go without being hit.