Felt dumb playing this game, I didn't realize you could shoot in the opposite direction and then rewind the bullet so that it goes past you and into an enemy until the last level. So, that felt way more challenging, by trying to shoot a bit off of the enemies and then bait them into the path of a bullet.
Anyway, that was a nice game. It feels like a beginner's game, but a really well made one.
By that I mean that you've got your core gameplay pinned down, but the polish is lacking :
-You start by telling the player to shoot but don't tell that you use the mouse/LMB. Which is weird since you though about telling that RMB is the rewind.
-The fact that there is no music makes the game seem barebone. Next time you should try to find a public domain BGM, at least. Same for the background, you should try to use a generic image if you can't draw, but it's always better to not have a simple unity background/basic rectangles as walls.
-There is no UI or infos about your progression. That can be a choice, but something like a message in the middle of the screen telling you at which wave you are when you start could make it a bit better.
-There is no way to quit your game beside alt+F4. Be careful with that, it's easy to overlook but it'll be needed in your more complete games. In the same vein, the ending lacks a "get back to the title screen" button.
-Finally, for the boss, you should be careful with your spawning of lesser foes. It's really frustrating to die because an enemy spawned on top of you. So, you should try either wait if the player is too close to a spawn point, or make the enemy appear a bit further away.
So, as you can see, it's all small details, but it's those details that makes a game go from good to great.
So, overall, really good job, even more for what seems to be your second game.
Next time, try to think more about the polish.
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Thank you for the detailed feedback!
Something I totally forgot was the UI telling you what level you are on (I added it in my last jam game)
I wanted to add music but I didn't have time to make it, and since I already made everything myself I decided not to add it.
Only after I submitted the game I realized that the boss level was too hard, so I'm reducing the difficulty on that a bit for the new version. And I did think about adding spawn blocking to the enemies at the boss level, but I was really running out on motivation, even when I made the boss level.
The reason I don't add a quit button is because I don't really use it in other games. I either quit with windows button + close window option on the taskbar or just alt f4. But now that you have mentioned it I will add it to my future games.