Extremely challenging game. The upgrade system was interesting, but on harder difficulties it is a bit unfair. Speed shouldn't go over 7 because it will start impairing the player instead of helping.
The main rule of the game is DODGE. Dodging is the only way to succeed because enemies get progressively stronger, but the player loses upgrades when hit, so it may become impossible to proceed.
Similarly, the "pacifist" medals rely solely on dodge because lives must be spared in order to crash-damage the bosses because bombs are not enough. The pacifist run for level 3 seems to be the hardest medal on the game and probably requires perfect runs on all levels (9 bombs and lives).
The final boss isn't that hard. Dodging the shots isn't exceedingly hard, in fact, bosses are the easiest part of the game because they have set rotations, unlike the kamikaze flyers that come out of nowhere. Using bombs negates projectiles and so it also protects the player during the "swap". It's much more likely to die in the road to the final boss than the fight itself.
Overall, a very solid game, which is seem on 440k+ views and plays. Good work. Almost perfect game!