It's a fun and challenging concept!
In terms of difficulty curve, I managed to learn how to reliably get past the first wave, sometimes at full HP, but couldn't get past the second one. I was thinking that through afterlife upgrades I would eventually have enough fire rate to better deal with the second wave, but over like 10 playthroughs I only managed to get 1 XP? I'm not sure if this is a bug or intended, but perhaps more granular XP (like 1XP per kill) and higher XP costs for the upgrades might help make players feel there's progress regardless of their skill level.
The music and the art are great (especially at setting the setting and mood) and the gun shooting SFX is pretty good. If there was one area of improvement it would be in the enemy death+SFX, they are good enough but I feel they could have a bit more oomph.
The one thing I found confusing was the HP UI being the revolver's barrel; for a long time I though that was your ammo and was confused when it stayed fixed while I shot bullets, and that there was no "bullet reload time" as might be expected of a revolver.
This is overall a pretty great game, there's great teamwork involved in getting everything fitting nicely together!