Mission 3: ReactorI've tried various methods and guides to get this working consistently. But this is the method I had the most success with, so sharing it as an achievement guide.
Helicopter #1 (North East):As you initially parachute in head towards the large white hanger to the East with the letter "A" on the side. If you land directly in front of the letter "A" you'll find a supply drop with a Precision Airstrike. Pick up the airstrike and hustle over to the North East helicopter in H4, use the Precision Airstrike on it.
Helicopter #2 (South East):Next grab the Cruise Missile found in square F4 and use the Cruise Missile to take out the South East helicopter in G7.
Helicopter #3 (West):At this point the armoured helicopter should now be patrolling the skies, ignore him for now. Make your way to the Mortar Strike in square C5, collect the Mortar Strike from the rooftop. While still on the roof, destroy the last objective helicopter in D4 with the Mortar Strike. This will create a perfect checkpoint on that very rooftop, which we'll use to destroy the armoured helicopter.
Armoured Helicopter:Now with a checkpoint on that rooftop, press
and restart checkpoint. This will reset the armoured helicopter, making it non-hostile and stationary somewhere in the middle of the map. Quickly run over and grab the Mortar Strike before the armoured helicopter spots you. Then run in the opposite direction from the armoured helicopter to stay out of combat. Now use the Mortar Strike on the armoured helicopter at range to destroy it. I found the tower in square B6 was a great spot to stand as the mortar would hit 9 times out of 10.