Troops in a valley fully respawn every hour. So if you hit a valley at 12:00 and defeat the troops, take back the resources, wait until 13:00, then the same troops that were there at 12:00 will be there again, even same hero usually. So you can hit again, get the experience, get resources, and another chance at dropping a medal. So no, hit at least an hour after your last attack hit.

Very good guide by the way!!! I have tons of medals now, been doing this a month now.