Wow hey, so much to love in this game! I'm surprised why it's not on the higher rated games yet! I want to go through some points:
1) The controls - simple and PRECISE. In very bullet-hell esque games, the kind where it gets quite intense, your controls really do you justice. The feeling of being able to move exactly how you want, evade projectiles etc. feels good. I will give credit to your good sound design in shooting, destroying enemies for having it feel tight and precise. Kudos for your amazing sound design!
2) A lot of good responsive feedback - you must be quite an experienced game dev, or have picked up good skills - as mentioned earlier, you did your game justice with a lot of sound design - something I am learning myself. I should be more specific in my wording of sound design - choosing sounds and implement them so that players know what happens all the time - ie. enemy dies from an explosion etc. Very good - commendable effort! Makes me enjoy it so much more.
3) Good art direction - With the little time, it's obvious that it will be difficult to source ratings and etc - but you made do with what time you have, and created a generally pleasant and cohesive experience! It was a pleasant to watch, and play from start to finish.
4) The switch between normal time and rewinded time is very smooth - I like it. The small invulnerability around the player helps smoothen that transition as well. Good game design!
Some pointers that I think you would want to consider:
1) In some FPS games, they show the 'reload circle' near the pointer to tell the player exactly when their reload finishes - consider implementing that but for your time mechanic. Instead of putting the clock to the side, you should put it exactly where the pointer is to easily tell the player without having him look far to the left. This becomes very important for high level play!
2) Small minor gripes with the control - as its a browser game, when you 'press space' to restart level, it also just scroll wheels to the bottom of the window, so I had to return to main menu to properly change difficulty or restart levels.
3) Consider adding a small grace period before starting levels - when starting on impossible, enemies spawn immediately which can throw players off.
There are more I'd say but I think that's encroaching subjective territory. If you'd notice, the only cons I have are only minor polish things I hoped you implement - which is great. The game as a whole feels good and nice to play. Lots to love and if you'd revisit this, implement more art, music, sound, levels and enemies - you could make a nice short game on mobile in the long run.
Love this! Cheerios.