First of all, enemies cannot damage people in multiplayer servers. Only the host takes damage. Clients go through the hurt animation but that's all. That's assuming you host in coop mode. If you use enemies in any other gamemode (from mce's), you can just expect the game to crash.

Second, JJ2 enemies are very easy to defeat, even when you have 1 heart. A swarm of them can be dealt with just by stomping over and over. Others just walk back and forth. Only a few really chase you to any significant degree.

If you can get around these two limitations you might have something. But I have no idea what you are planning.

In any case, make your level worth with 16 players but allow any amount. Music doesn't matter. 2 I guess, because it's better.