This achievement requires you to shoot down 5 helicopters in the Mi-8 helicopter battle Instant Action Mission.
To access the mission, select single player from the main menu. From here, select instant action. The Mi-8 helicopter battle is the second mission. Select it and make sure you're on the easiest difficulty setting and ensure all collisions are disabled. Next, you'll be able to choose the helicopter you want. I selected the Mi-8 and loaded all three of its weapon slots with S-5 rockets.
When you start the game you'll see the helicopters marked in the distance. No need to shoot your rockets yet as they'll fly close and swarm you. Once they do try and hit them, adjusting your aim if necessary. Remember for these rockets you need the green crosshair to be aiming at the target. Hit 5 and your achievement will unlock.
I also earned the achievement for shooting down a helicopter from 100m with an S-5 rocket but I was a lot closer than 100m, so you may earn this here too.
The Rockets master achievement in Air Missions: HIND worth 119 pointsShoot down helicopter with an S-5 Rocket on distance greater than 100 meters in singleplayerYou also only have 5 reloads. So you have 3 rows of 36 rockets, which is 540 rockets, which should be plenty as long as you average about 105 rockets per helicopter.