I'm on mission 16 or 17 now and I only have 3 heroic wins so far.
I'd either restart or at least jump back several missions. If you're following RD's outline of the missions, you should have an idea of what it takes to Heroic each mission. Keep replaying them until you get Heroics on most, I wouldn't move on... This also impacts your money and Patton upgrade points, so you need all of them to make sure you're prepared for late game.
Keep in mind a couple things:
1.) Your guys (preferably Snipers or Commandos) can shoot way farther than their "range" is defined. Just line up the black line so you are sure they'll get a hit. Also, Bazookas can do the same thing, you can shoot tanks all the way across the map if you line it up right.
2.) If you aren't in range or in a good enough spot to shoot someone, be sure to "ready your aim" before changing turns. This will make it so if any enemy walks into the defined area, your guy will shoot them automatically (provided he has shots left still).