The game looks very polished for a jam entry! I really liked the attention to UI, sounds and overall palette! And the enemies are good incorporated in the overall setting.
I found it really easy on easy difficulty so I quickly switched to hard. The choice of guns was good balanced in my opinion but railgun and machinegun were my favorite.
For some reason it was hard for me to understand the differences between the time, health, energy and XP. I believe my first impressions were that when you collect sand you gain XP because you take something and you see the big XP bar above to go up. So my attention went to that big and wide bar and I didn't notice the changes in the small sand bar which actually is more important.
I think I would prefer it to be the other way around. Or maybe you can pin sand timer and sand bar to the player. This way you would have all the important information in the center of the screen because the user is constantly switching attention between player and enemies. For some reason I also believe I would prefer if I gained XP from the fact of killing the enemy (with a simple message "+100XP" near the player) while the sand were only connected to time, health and gun energy (maybe it is already this way but for me it felt different). Right now the game setting doesn't explain intuitively why all these things are connected. You have sand everywhere but the sand from the enemies is somehow valuable. Maybe you can change it to some holy water (like the color you use for the XP bar). Then you would use holy water to sustain your health and also you use holy water for your guns (regular water under pressure is very dangerous). And your sand enemies could use holy water as their soul. That's why player would need to collect it for themselves before it dissapears from the sun heat. Anyway... My imagination is taking me over :)
Overall I liked your game and I'm looking forward to see where you can go further with it if you will
Good job and good luck!