I just completed this achievement and thought I'd share my observations and techniques for achieving the Heroic Victories (all in one playthrough!)
You will get this on your way towards:
To gain a Heroic Victory, you need to be:
1. Accurate with your shots (don't try to make cross map shots with your riflemen)
2. Complete the mission in as few turns as possible
3. Avoid taking health damage (a couple of shots is surmountable)
4. Absolutely avoid losing soldiers
5. Try to kill as many enemies as you can - but don't sacrifice HP to do so
I found the first half dozen missions really quite tricky to get Heroic Victories, and my original intention was simply to play a second playthrough with my prestige points all maxed out. However, once you're able to unlock the B52 bomber and with highly experienced snipers, this becomes a whole lot easier.
I'd also note that the infiltration missions are particularly hard to gain Heroic Victories, so don't be too concerned with them.
Once I had unlocked the B52, my approach was to use 3/4 snipers, 2 bazookas, 2 commandos (grenades are very helpful) and the B52. Often splitting your team up into two groups and going opposite routes round the map was the best technique.
The best approach is:
1. Absolutely play on picnic difficulty. There's no reason for making it harder.
2. Lead with your snipers - their extended line of sight is incredibly helpful.
3. You can easily kill units outwith your line of sight limitation - snipers and commandos are very accurate, even at range.
4. Bazookas can easily kill tanks that are visible because of snipers' sights.
5. The B52 can also kill units that you can't see - specifically where units have retreated just outside your sight lines.
6. Restarting is your friend - get an idea of the map and enemy layout and replay.
7. Fuel is allotted per map - you can't pick it up or increase it. Always use the B52 when you can.
8. Don't use tanks or any other aircraft. They aren't particularly helpful.
9. Use cover and the X button to crouch your units when switching turns.
10. Defend missions are by a distance the easiest - a medic can also be helpful.
11. The B52 can have 10 bombing runs per mission - you don't need to be particularly sparing of their use.
12. Flank wherever possible. Straight down the middle is rarely the best option
13. Small movements - your maximum distance in one turn can be done in 14 moves or 1. Small movements mean you won't be surprised.
14. Grenades are absolutely your friend.
This is the order you should use your prestige points (also note that Heroic Victories reward 4 points, Minor Victories only 1):
1. Tactical (numbers of shots per turn)
2. Charisma (prestige per mission)
3. Logistics (ammo)
4. Offensive (only really helps explosive damage - bullets kill everything anyway)
5. Defensive (you should avoid taking damage for Heroic Victories in any case)
6. Prestige (your units gain experience far faster than this is needed)
This should set you up and you'll be on your way. A pretty easy achievement once you get used to the controls and techniques.