I farm medals via valleys more than from NPCs. From my experience, I personally think what units you send (amount/types) has absolutely nothing to do with the drop rate.

Just a few days back, I was training my highest attack hero (around 156) by hitting the same level 10 valley with Cavals and Cataphracts. I got 3 honor medals back to back...I would love to post those reports only if I still have them. (so proud of my horses )

Anyways, my conclusion is that just keep trying...Hit NPCs and valleys as many time as possible. Nothing in life is easy, so don't give up easily.