I haven't played with anyone else yet so I don't know if this is already a feature, but I think scaling the number of enemies based on players in the match would make things easier/balanced. Playing as a single player on even just the first level is really frantic, especially with like 10 kids kicking 16 different gravestones. I feel like on the first level you should be able to knock out all the kids pretty swiftly without losing any gravestones. Also, just scaling back the difficulty early on will make the game last a little longer.
Side note: Can the box for picking up enemies be bigger? It's kind of annoying after a KO that I have to look directly at an enemy to pick it up. Maybe I'm too used to the FPS "run over an item and press E" thing.
Otherwise, great job on the game! I love it.