I found a little gem of info in a writeup from 11 May 1979
"
To Ensure a terrain mask for helicopters, select a route on which higher terrain lies between the route and the known enemy position. Mask the helicopter from both electronic and optical weaponry. When terrain or the location of enemy weapons restricts masking from both systems simultaneously, mask the helicopter from the electronic system. This situation occurs when the enemy's optical weapons are strategically located to fill gaps where his radar cannot detect low-flying aircraft. Visual detection at night is difficult for optical guncrews, even when the helicopter is not masked; but electronic detection is not affected by conditions at night."
Lieutenant Colonel Richard L. Phillips, USMC
Another from the same writeup
"
The recommended tactic for a helicopter pilot under fighter attack is to fly directly toward the attacking fighter to cause the fighter pilot to deepen the dive and increase airspeed. Just before anticipated engagement (...Most fighters begin firing from 1,500 to 1,000 meters out...), make a sharp turn either right or left but in the direction from which the helicopter pilot will best be able to keep the fighter in sight. Should the fighter stay on the helicopter in the turn, another sharp turn in the opposite direction should end in disengagement....
Mountainous, rugged terrain will normally force a fighter to make steep diving attacks......
... Avoid tight geometric formations because they prohibit effective evasive action ..... Trail formations may provide a fighter pilot with an opportunity to hit all helicopters in one pass. Loose, staggered, or free trail formations are probably the best formations to use.
And more
"The most vulnerable point of a mission for a large formation of helicopter is to be caught by fighters while on short final approach to a landing zone, because not only have the helicopters lost the ability to maneuver, but rockets and bombs will also be effective against the entire formation."