Thanks for your feedback, actually I've been play tested this game over hundreds of times, so I'm pretty numb about the difficulty of the game. For the boss battle, the boss will turn vulnerable when it's preparing to attack, you need to shoot it while it's attacking, I vary the gun hit effects and wish the player to find this out.
I'll take your suggestions and make a new update, probably adding tutorial to tell players to attack the boss while it's attacking, and lower the difficulty by increasing player's endurance (So it takes more time for pests to take you down).
Thanks for playing my game!