See also Evony Guides

Cross Medals increase a hero's loyalty by 5 when rewarded and is used for hero persuasion. Beside as award given to heroes, it can also be used in the Persuasion of rival heroes.

You will have a greater chance of obtaining these medals by attacking Level 1-5 valleys. 
Also acquire a Junior Medal Box, either from the Shop or from the Roulette Wheel, or purchase for a price of 80 Cent(s).