This desparately needed a movement tech dude, the enemies attack too fast, you were trying to emulate the 16 bit movement? But the enemies should have some time of delay between getting to attack position, spefically the porcupine, and the knockback, is very bad, since games from that era didn't have them to begin with, and it can push you into other enemies killing you, the random enemies also can easily sorround you, making it nigh impossible to win, the platforms, though technically a good idea, end up hindering the player more than they help, the flying enemies, don't have a height cap, so they will just go out of screen to not get attacked, they can only be attacked while and a little after they attack, which makes attacking them harder, there's no invincibility frame for attacking enemies, or getting hit by enemies gives very little invincibility frame, so if you are sorrounded either due to random enemy spawn or just due to the inability to kill them fast, you will just die. And the double jump help, but not that much since the entire jumping is too fast, and makes it hard to make tiny adjustments.
Also the best method to get to the boss was to just spam the attack which isn't really a good gameplay loop.
If the player had at least a dash that makes them invincible, that would have helped massively, or just invincibility frames right after attacking or getting hit would have helped a lot, should have checked games from the 16 bit era for the attack and movement and enemy movement.
Also since you get knockeback after hitting enemies, the enemies should probably get stunned too, so that the player has time to get back and attack again without worrying about the enemy immediately attacking, unless of course the enemy is a boss, in which case them not getting stunned makes sense.
All in all decent idea, but just the execution wasn't the best.