This was a lot of fun. You had quite a variety of bosses here, and they all felt very different from each other. I managed to beat them all, and they were tough little fights. I'll rank them from least to most difficult for me:
1. Druid
2. Faker
3. Ice Wizard
4. Take Shrooms
The Ice Wizard gave me a tough time for some reason haha, but I managed to beat him too. Fun fight though. Really, all the fights were fun and had a good variety.
The gameplay was cool, and the fireball felt good to land. It had a nice impact with the screen shake and bassy boom SFX. The shrink was super helpful when I was in a bind too. I ended up using it as my emergency "get the heck out of this area" move. The condensed/shrink fireball was strong, but felt balanced nicely since you still had to commit to using it well. The music was fun and epic, and the pixel art looked good too. I especially liked the boss art.
Fun entry, and the cowboy wizard idea is just a great setup.