Arguably the hardest aspect to this game will be the achieving the 5 and 10 heroic victories on 5 and 10 different missions in one playthrough. Heroic difficulty for starters is the best rating you can receive upon completing a mission. What contributes to rating? This:
Number of shots hit compared to number of shots fired
Number of unharmed to the number of wounded to the number of killed
Number of enemies killed
Number of turns to complete the mission (the more turns the less likely for a high rating)
IMPORTANT! Please note that these 5/10 wins need to be done in one campaign playthrough which is 21 missions long. If you do not get 5/10 heroic victories in one playthrough you will need to do another playthrough in order to get 5/10 new missions to the Heroic ranking.
But which missions are best/easiest to do in order to get these heroic ratings? It's up in the air. Some people are much better at the stealth missions (I personally hated them and simply rushed--hard through them). Get a good feel which missions you are best at, and just plan ahead. Notice the different symbols on the map when you are moving onto the next mission.
From the deadly dog at trueachievements