Very neat! There's a lot going on all the time, but it never felt overwhelming or unmanageable.
I'm generally bad at multitasking, yet I was able to move the robot and fire on enemies while still guiding the drone to pick up crystals pretty effectively. The controls are well thought out to make everything feel very intuitive.
The enemies have the right amount of health, taking enough hits that you need to focus on them, while not so many that it feels like a chore. Even when there were a lot of them on screen, I was able to fend them off thanks to the very quick fire rate (and the ability to hold down the button to fire!)
The music is suitable tense. The sound effects fit perfectly. The one thing I would say about sound is that the robot's gun does get repetitive with the amount of firing needed. If you altered the pitch up and down slightly per shot it might help with that.
It would also help if the total number of crystals on the level was displayed somewhere at the start; if it is I missed it. Otherwise the player needs to walk over to the alter with at least one crystal and push E twice to get the total they still need to collect.
Really fantastic and fun game overall!