Hahaha!
I'm super happy that you made it all the way to the end!
In hindsight, I should have made the boss room a separate one. But instead, I just made it so that you could only pass to the "thank you for playing" level (the next level) if you clear that entire room (boss room + enemy room). So unfortunately you didn't see that room :(.
Tell me, did you have any issues when the vanishing enemy appeared? Game froze at all? If not, what browser are you using?
Thank you for playing!!