Alright, you specifically asked for real / harsher feedback so here it goes.
So first and foremost, with some changes I think the game has potential to be very fun to play. Shapeshifting is cool and switching to different characters with unique abilities could be a very encaging game loop. With that in mind, here's some things I would personally change about your game.
#1 Controls
The controls are very janky, switching quickly from player to knight is not intuitive, and you didn't explain the full controls in your controls section, I had no idea you had to press E + Arrow keys to switch shapes so I almost gave up in the first level. Also you never mentioned spacebar activated knights attack.
I would either have E open a menu where you can click an option to switch to said shape, or maybe just have number keys switch to specific shapes (1 to human, 3 to scientist 4 to knight etc.)
#2 Level Design
I am specifically going to talk about levels 1-5 since I couldn't make it through level 5. I apologize if you addressed any of my concerns in following levels and I missed it.
Level 1 you could have had the player start as a different character so he would have been forced to use shapeshift to player, teaching the mechanic before level 2.
Level 2 was fine, main issue is the controls were unclear which almost made me quit right there.
Level 3 the collision was really bad, I kept getting the player stuck constantly on my replays trying to get through level 5. Also sometimes the first enemy hits you and sometimes you can walk through him (the one who is in the hallway right below the hallway you're supposed to go on). An easy fix for the collision would be to make the players collision box smaller, idk what shape you made it but making it an oval of sorts would also help with it getting stuck incase you had it as square.
Alternatively you could have made the whole level a little bigger and give the player more room to move, which could have helped the fact that the maze was too easy and you could instantly figure out the path to take, ignoring most of the level.
Level 4. Best strategy was to just run straight to the door as player. I feel like your goal here was to introduce the scientist class? If that is the case I would have made maybe another maze, where some parts of it you had to switch to knight to kill enemies, and other parts there were too many enemies so you were forced to use scientist. As it stands you could just run to the end as player and skip everything.
Level 5 Way too hard. Remember that you played the game for 48 hours, where as the player hasn't at all. You should design the game to be a little bit too easy for yourself, and that will generally translate to a comfortable challenge for new players. Also if you have to have a super difficult level with a specific solution, let the player restart that exact level. It was miserable going through the first 4 levels over and over again just to instantly die on level 5.
Besides that, there was never a spot to use the rock. If you turned into a rock the enemies would just surround you and the second you turn to something else they insta killed you. I would either remove the class, make the enemies move away from the rock so you could turn into something else after, or give some sort of armor buff to the knight after shapeshifting out of it.
Final Thoughts
You submitted the game 9 hours before deadline. If I was you, I would have used those 9 hours to watch "The Art of Screenshake" on youtube, and use the remaining 8 to implement as much juicyness into your game as humanly possible. Makes all the difference in the world to how it feels to go through it.
I am not an artist, so I can't help you a lot with the art. One thing that you could do outside of just jamming in VFX which is what I do, is just adding some simple gradients into the world. Having different shades of a color looks a lot better than having just one solid color, and doesn't require a lot of effort.
I hope you find at least something I said helpful, and if I was harsh about anything remember you asked for it (= Good luck on your game dev journey bro!