Walking up the staircase doesn't do it anymore, at least for me. If I don't kill those few enemies who hang out around back of the entrance topside they won't re-spawn for me when I go back down.

From what I've found, each time you run back up you MUST shoot the 2-3 enemies behind the entrance on the top side for the respawns to come back immediately. If someone else has shot them your going to have to wander to kill at least 2-3 enemies. I swear it's ridiculous how much they want you to wander around and find enemies when there is nothing to do so who cares if your killing the same enemies as they keep re-spawning them anyway!