I just did OMF NM HC with a friend, we both used 4 heroes using the xbox pad emulator splitscreen. On wave 13 my huntress died like 8 times and respawned despite the HC mode and suddenly the wave just jumped from 13 to 14 midwave. The mobcount didn't seem to change but I'm not certain about that. When the wave ended all my defenses got despawned and mobs kept coming so we just went to tavern.