I have gotten as many as 13 medals from the same valley in a 1 day period.
Used to keep track, you do need to wait about 10-15 min between attacks minimum. Also sending in an attack hero with high intel or a hero with high intel and moderate attack works best.

Troops attacks can range for anytype of units, some only scouts and some only cavalry. Mixed units seem to work best for me, but if you need to take a valley fast just send scouts, they are cheap.

You do not have to take over and abandon, the drop rate is about the same as just a straight attack. If you want them to have full troop strenght then you will need to abandon each time.
Max