It's a pretty cool concept having time specific changes to the game. One thing I would suggest is to have the monster fire a variety of bullet patterns instead of firing in the same direction each time. I was able to easily defeat the monster by just standing in a spot where the bullets miss each time.
You could probably try setting an array that determines the angle of the bullet spread each time the boss fires.
Ex: [90, 30, 60, 20] degrees apart each bullet