Quite a good strategy for people new to medal farming. The layering is a very good idea, but you should really use much more transporters for looting absolutely everything in the valley or NPC. Try doing this with NPC's, since the drop rate is much much higher. Just a suggestion, NEVER use ballista to farm as they will slow down your attacks by a giant amount. You could send more than three waves of an archers, warriors, cavalry and scout combo for every combo that includes ballistae. The whole purpose of farming is getting medals as fast as possible.