I just completed the first level for the 2nd time after a few tries. I have to say it's VERY dependent on random chance for crate health and XP drops in its current state. From the left, the 2nd, 3rd, and 7th weapons are quite underpowered starting out, and there's even less reason to upgrade them later on, so focus on the others most of the time.
To the dev: consider showing the change in health on the NEXT 4-tick interval instead of the PREVIOUS one. This might require you to constantly monitor for changes in the number of enemy keys but it will give feedback sooner to the player on how their health will change.