This review has more criticisms than praises.
I'm not trying to crap all over your game, which obviously has a lot of work put into it. I'm hoping my input can inform some game design choices in the main project you mentioned. Most of my grievances deal with the information presented to the player, and how level design can inform or confuse the player.
I quit after dying once at the first boss. If an enemy is going to cycle through the same pattern and has to be "killed" more than three times, it needs to have a health bar or else the player is going to question whether what they're doing is really working at all. I have no idea if this boss has to be re-killed five times or twenty, and I guess I'm not going to find out. It should either have a different pattern (speeding up isn't enough), a health bar, or change sprites each time you re-kill it to show that you're making progress. Or just die after 3 cycles.
"Is that water? Is it quicksand? Is it safe to go in there? I could just jump over it..."
Went down into the water, air bar hit zero before I got into the little breathing area, but nothing happened. I didn't bother to check if waiting around even longer would deal damage or kill me outright. I would make the air bar take away smaller chunks rather than 1/4 at a time. Seeing how far I still had to go with a little sliver of air really made me think it was designed to kill me until I could come back later with an air tank or increased lung capacity or something, but nope. Got to the breathing area, got the Melee Attack, and got out. And then I jumped over all the enemies because there's no known incentive to kill them, and because using the melee attack would require putting myself close enough to take damage. So I went out of the way to put myself in more perceived danger for a prize that I had no use for. Also noticed that this was the only water section in this part of the game.
At the first bridge section, I was able to physically jump up and touch the switch several times before I acquired the gun. Up to that point, all the game mechanics revolved around the platforming elements. I know shooting switches is a common mechanic in games, but it's more prominently-used with energy-based weapons. I'm sure a lot of people immediately got the gun and knew to shoot straight up at the switch, but since I tend to overanalyze to a fault, I didn't arrive at the logic of "shoot this manual override switch with a bullet" until I managed to jump up to the switch and press Down (and again with Space) and have nothing happen. Maybe make the ceiling high enough here that you can't even get your body close to the switch, especially when you put the switch up between two parallel walls in the same room where you get the ability to scale two parallel walls.
In the room where you get the grenades, I was so distracted by the box of grenades that once I got to the health station wayyyy up in the air, I didn't notice the exit path tucked away up there. This kind of misdirection is great when you want to hide the path that takes you to secret content, but since the exit is there, it could use more "level" on the right of the health station to indicate that there's a reason to travel to this side. I assumed (again, dangerous) that the grenades had something to do with the face boxes that were introduced before the grenades. of course that was not the case, and after leap-of-faith killing myself off the right of that screen, I explored around until I found that exit up top again.
I assumed it was intended to be at least a slight challenge to land on the platform to collect the grenades (it was, and it took me a couple tries) but it was less of a platforming challenge and more of a "fighting against the controls" challenge.
Dicked around with grenades trying to lob them up into the other bridge switch (which, again, is just low enough to suggest that one MIGHT be able to jump up there) and then made my way to the
boss.
I assume you're not going to edit and resubmit this, but I hope you'll take it into consideration for next time.