The best way is using UCAS-D. Pilot of the helicopter will lose control immediately. The way to save the helicopter depends on the skill of the pilot.
Better use UCAD-D when u hide in build near it. When u see it. When u can shoot it after pilot lose control.
Use light machine gun to destroy it.
Second way. If u don`t use UCAS-D, or u don`t have points.
Always shoot at helidrone (better use light machine gun). Don`t stay on open place like idiot. Hide in houses. Shoot throw windows.
Don`t forget use heavy 12.7 mm machine gun on the vehicle. Create a tight fire. And use RPG by yourself or teammates. If u will use only rpg or only machinegun it will be not so effectively like use both of them with your teammates. Cos pilot from bullet hits loses concentration and may not notice a shot from the RPG.