Neat game, I had fun! Despite the amount of info in the tutorial, I think it was still explained it quite well and it's good you could break it in some parts throughout the gameplay rather than just having it all in a manual.
Other than what's already been said in other comments about the enemies fading out a bit too quickly right as they appear, I think my only suggestion would be good to limit how much the player can hit them at a time before they disappear. I realized I could just spam click if I had my mouse on them fast enough, and this would grant me at least 3 or 4 clicks and making it far easier to kill them. Though it still wasn't trivial, you might prefer to have a cooldown or make them disappear more quickly if they're clicked (so as to limit it to one hit per appearance).
Either way, great job!