Get them before they stand up, especially the big ones when you get to stage 13. The skinny guys can also take a lot of shots if you don't knock them down first. If you clear a stage using the shotgun, wait until he reloads it before clicking to the next stage. And if you think you'll clear a stage with the shotgun before its empty, go ahead and blast away so you can get a reload before the stage ends. You always get a full pistol when a stage starts, but the shotgun won't reload automatically. One other tip - if you have at least 4 guys on their feet in the higher stages, its time to use the shotgun.
Maybe we're more than what we think we are...
...or maybe not.