It's something I made in a couple of days to get back into gamedev. I agree I should have playtested more with stuff like this, especially with killing enemies in different order than I usually do or not killing enemies at times. It's stuff I have to test with level design in general. I spent most of the time on making the art, it's something I have struggled with past games, and I liked how it came out. It's stuff that comes with practice. With the fireballs, I didn't want it to be normal and to be something unique or at least different. It's something I was testing out. The fireballs not going immediately is because I wanted to try out traps you can lure enemies into, so it would be an extra dynamic for players. If more people don't like it, I'll rework it if I make a sequel. I didn't think of adding check points with the game only being one level with a boss, if I make a sequel there will be some checkpoints.
Thanks for the constructive feedback. I'll definitely take note of it for the future.