the trick with the shotty is when you know that your about to kill the last guy on the level you kill him and quickly shoot out all of the remaining shoots, then the guy will reload while the level complete text is up and thus you have a full clip for next level
i couldn't say thats the BEST advice but yeah it would help
headshots mean nothing in this game lol
the ones that actually do something when you shot them in the head, (head falls off) they just keep walkin like nothing happened lol