Yeah, had the same problem, have no idea what is going on. Enemy references are not updating at the tower target list. Maybe some wrong variables from the previous bug cause this bug. Don't know yet.
I left the world through the door to cancel the medium game when it is running, this way you can reset all variables. Then i played twice in a row on medium. No problems with wave 15. But my recommendation for you is to use the console with the "KILLALL" command. Then you can proceed with wave 16.