Overall a fun game with great sound design. I enjoyed seeing the bullets bounce, that's not something I usually see in bullet hell style games. It adds another layer of complexity.

If you wanted to improve, I suggest resetting the player health between each boss encounter, or giving the player some sort of choice like 'more powerful weapon' or 'return to full health'. This could create an interesting choice for the player and add a lot of replay value. Secondly, add more juice, add in some particle fx when you land a hit or get hit, maybe some screen shake too.

If you keep working on this, let me know and I'd happily try it again!

Damn the soundtrack on this game is excellent, props to the sound designer here! 

I have a little bit of constructive feedback:

• make the player shoot a LOT more quickly! Make it so I can hold down the fire button, reduce the amount of damage per shot, increase the bullet speed, and it'll feel much better :)
• the art is decent, but I think a little more contrast is needed. Make the background nice and dark, that way all of the important visual elements (the player and the bullets) will "pop" more and they'll be much easier to see! I'd also lighten the enemy bullets :) you're using Unity, too, so there's no excuse for not installing the post processing package and adding bloom ;) 
• more feedback! When the player gets hit, the screen should shake. When the enemy gets hit, they should flash white.
• the menu is straight up broken xD I'm pretty sure you've got something covering the buttons - I'd make your canvas scale mode set to "Scale With Screen Size", that way it'll always look the way it does in the editor :) I'd hazard a guess and say you've got it set to "constant pixel size", and in the build that has meant certain things have moved around and the button is being covered.
• I don't think there's an issue with the enemy bullet size, I think your issue is with your player's hitbox. Make it smaller than the visible bounds of the player! In most bullet hell games, you can be hit without dying on around 50-60% of the player sprite, with a small glowing bit in the centre that actually gets hit. This makes it feel like you had a "close shave", rather than thinking "ugh I'm sure I dodged that"

Other than that, though, it's a really good entry! I think you've got a fantastic little concept on your hands there, and with a little bit of polish I can see it being a game I come back to! Well done :D

Here's a video that illustrates a lot of what I was talking about:

Very nice sound design, and the general gameplay is alright, but there's room for improvement:

• The main menu has an option for Controls and Instructions even thought only the Start button works.
• It would be nice to have more diferentiations between bosses, like different shape bulellts, different wall and room colors e.t.c. I would try to give more personality to every room.

Game isn't bad, but it's a bit difficult. Bullets are too big and it's hard to dodge them.
Also not sure why player is spawned in the same room before each boss and the only difference is boss.
Other than that I think the idea is fun and with a little more work it could be really good.

Nice simple game, movement felt smooth. Projectiles are a bit too big and hard to dodge. Taking damage on purpose to get invincibility was the best strat for me. Very good work to the team!