I don't think changing the AI would make the game that much harder, by it self that is.
I could see a few bosses being more difficult, but they would still have the same attacks.
If a real difficulty patch were to be made, I don't care either way, it would have to change the attacks and bosses themselves.
Either making the projectiles faster, different patterns of said projectiles, do more damage, less time in between attacks ect...
The Z-saber doing 7 damage to most bosses, in a single charged slash and not the 3 slashes,
seems really strong to me, considering Zero already has 2 full charge shots before the slash.
Going with 4-5 damage seems more reasonable like on the medium bosses, but for all bosses.
Great job with all the work so far, looks really good. I like the horizontal bars.
After you have gotten Zero to where you want him to be, I think a good addition would be how weapons affect the Z-Saber.
A fully charged shot would empower that weapon into the Saber somehow, giving Zero an other unique set of moves instead
of just having the charged weapon be the same as X's charged shots.
One question though, do you have plans to change Zero's colors based on the current weapon like X?