Okay, just beat the game with maxed out upgrades.
Intially I was gonn complain that I ant max out everything since enemies now dont spawn after like level 10.
But, likely not intended at all, the following happened:boss spawns, monsters stop spawning.
I kill pretty much all monsters.
However aside from the boss a single immortal bat and an immortal skeleton persist.
like you can "kill" them in the sense of shooting them and they drop points or hearts. but are up and about nonetheless. so yeah, I was getting chased by them for a while while running in circles shooting the skeleton behind me leaving a trail of points that I would later trail along once I got back.
did that for quite a few round before everything was maxed out.then did some run and gunning around the group to hit the boss with a few stray shots and take him down.
fun times! :-)