This boss shows up at the end of Wave 3, once you kill the initial attack, and the subsequent group a message will count down to it's arrival. If you kill all other files before it arrives you can use
to see which side of the level it is coming from.
The fly itself doesn't shoot but simply flies slowly towards the cupcake. You can fly along with it battering it with candy till it dies. If possible use the explosive yellow flying sacks
to hit it with fireballs, this may take a bit of practice to get used to the timing, but I managed to get one to explode right by it and killed it in one without firing a single supporting shot.
Kill it for the achievement.