Age II medals are easy. I can get 20 medals a day ranging from cross to justice. National are harder. Here is where you find them and what you need to use:

Cross & Rose medals Hit level 1-6 valleys with warriors.
lv1 valley = 80 warrior
lv2 valley = 160 warrior
lv3 valley = 320 warrior
lv4 valley = 640 warrior
lv5 valley = 1200 warrior
lv6 valley = 2500 warrior.

Lion & Honor medals. Hit level 1-3 NPC's.

Courage & Wisdom medals hit level 4-6 NPC's.

Freedom mainly drop from level 7 NPC's.

Justice mainly drop from lv10 NPC's.
This info is very accurate as of now. Lord knows Evony will change the drop patterns again. They always do.