This game is quite good from a mechanics standpoint, though it is clear that it was not thoroughly bugtested or playtested to balance encounters or exp growth.
However, the final boss is absolutely indefensible trash. It has nothing to do with the gameplay leading up to it, the hitboxes for you and the enemy are unclear and inconsistent, and half of the boss's attacks are unavoidable because you can't see them coming or she spawns on top of you and instantly kills you.
After getting a run to hit her 8 times and not winning, I decided to find a better use for my time.
Edit:
I beat her after cooling down and giving it a couple more tries. It takes 10 hits to win and you need some help from rngesus to make sure she doesn't hit you with a buggy undodgable move, but it is doable.
My personal strategy was to sit at the bottom middle of the battlefield, attack the claws, shift slightly left or right to avoid the ominous ceiling wiggles, and to prep to go up immediately if she spawned in laser mode to my immediate left or right (this is the part where you need luck that she doesn't spawn a few pixels up from the bottom of the screen, or it becomes basically undodgable). With this strat, I'd say 50% or more of the time she will spawn just enough above you for a claw hit to be possible without being lasered.
Her hit box is by far the most irritating and inconsistent part of this. The only consistency is that her head and legs are not part of it (but your head is when it comes to the death laser). You basically need to land the claw's slash animation on her torso for the hit to (maybe) register, and it needs to be immediately after she fires the laser (she is invulnerable as she fades out).
The dev was clearly going for an epic final bossfight but didn't take the time to learn how bullet-hells need to be built in order to support this level of difficulty.