Would you believe me if I said I didn't playtest much (especially later into the game)? I know I would! (but then again I made it...)
From my experience at around level 30 the enemies only then start becoming a challenge on all fronts, exept health. Why? The enemies don't have enough max health and you gain wayyy too much damage with damage and attack speed upgrades (but attack speed kinda sucks, because of the heat increase being greater than the attack speed increase). Good news: I took some time to balance it and make it more enjoyable. Bad news: I did it after the submission period ended, so the balance tweaked (plus a little bit extra) version will only be avaliable after the rating period has ended.
I'm glad you enjoyed my little creation -- thank you!