I really liked it!
The upgrade system was awesome and the classes felt very distinct. The sky's animation was wonderful and really set the tone, and I think some props on the ground, like trees or bushes, could have made the vibe even better.
I really liked the detail of how many shots you have left being indicated by the number of orbs in your hand. It really was a nice touch!
The only problem that I felt was quite important is not being able to tell when you get hit and what hits you. A simple sound effect or screen overlay may help.
Other than that, after playing for a while, I felt like I was getting swarmed by a thousand enemies and that there really was not much I could do about it.
If you ever find yourself coming back to this, consider adding AOE abilities or something like that. It would really be satisfying seeing a giant mass of enemies getting demolished.
I also found the time-slowing mechanic really cool. I always love it to see it in games, although I feel like it is kind of underutilized.
I really liked it!! I am not good at FPS's, but I think having it magic-focused is a very nice change of pace from the usuals!