I was having a terrible time trying to get roses... this is what I did to FINALLY start getting a few...
lvl 5 valleys (mainly flats and lakes gave me a rose)
sent out many waves from 2 cities daily.... would get approximately one medal (either rose, cross, and occassional wisdom) in about every 40 attacks to a level 5 valley. Using 2 cities, sending out 7-9 attacks from each to a different lvl 5 each time, approximately 6 or 7 times a day, I was able to get about 2 rose medals per day. Volume is the key.... hit lots of different lvl 5 valleys as often as possible.
sent for attack:
I used the horse units for speed so I could get more attacks out faster. I would lose some (about 40 per attack depending on valley forces), but loss was worth it to get the rose medals.