Nice game! It's simple but fun. I'm going to have some feedback and recommendations here but it's only because I like the game and want to help the dev out!
* The pixel art is blurry when seen in game. I don't know how you would fix this or where the problem is because I don't use Unity but here's an answer online that might fix it https://forum.unity.com/threads/blurry-pixel-art.665701/
* The controls are a bit odd and unintuitive, I'm not sure why you would choose to have the fire direction independent of the character. Maybe try having the character fire in the direction it is facing, or if you want to have independent firing use the mouse instead of the keyboard to control which direction to fire. Maybe this isn't really a problem or difficult to solve, but to me the controls could be more intuitive.
* Your bullets go through the enemies and hit others - is this a design decision? Maybe try not having the bullets go through enemies and see how the game feels. I don't really know what would work for this game so try out different ideas.
* I didn't know that to charge the boss you had to move out of the summoner zone... Maybe add a short descriptive text at the beginning describing controls and info like this.
And here are some features that could be interesting if you expanded upon the game:
* Additional enemy types would be interesting, such as a ranged unit that would move around the arena and shoot out projectiles
* Maybe make it so you have to defend the four summoners and if one of them dies the summoning slows down.
This is a great game, and I hope you liked my feedback and it helps you improve. It would mean a lot to me if you gave me feedback on my Weekly Game Jam 102 submission, Darkness Defenders. https://nicholasc.itch.io/darkness-defenders