Maybe press the flag a few times. Press the arrow keys to move and space to shoot. Press V for a bomb. Down arrow key to block attacks. Press 1 through 4 to change difficulty. Difficulty changes boss HP. Number 5 turns on survival mode, where you have to survive versus an immortal boss for an unlimited amount of time. This game will take time to learn and beat.
Thanks to @Hedgehoes for recommending a nice idea that made the game feel less like a clicker, and more like a bossfight. Thanks to @RylanKnoop66 for helping with a few bug fixes.