This game was actually pretty fun, good even, especially considering the time limit.
There are two suggestions: One, I think that the flashlight mechanic, while actually very neat in design, could've been implemented better. I would personally have opted to make it only be active while holding the right click, the right click toggle is not a bad idea, but having it automatically toggled on kind of painfully teaches the player about the battery by letting it die immediately. Then, when they pick up batteries, they almost act like a temporary light buff than a resource to manage. I would've opted for the holding personally, since the battery is so quick to deplete, allowing users to toggle for brief moments, instead of needing to aim, shoot, and simultaneously toggle the flashlight on and off.
The other suggestion: The flashlight didn't seem to have any effect on the final boss, and while I can grasp that one shouldn't stun the final boss just for managing your battery, perhaps you could've reduced the boss attack speed for a moment after being exposed to light, allowing for expertise with the flashlight to be most capitalized on.
Not so much of a suggestion, but I might consider making the large slimes have some kind of effect, as it was, they felt more like objectives. They probably were objectives, sure, but even so, as what appears to be organisms, maybe make them a spawner, to make it a bit more urgent to kill it, instead of being able to kill your way through the enemies with careful kiting, even without a flashlight.