Battlefield 3 Cheats
there are several ways that seem to work:
1. remembering the position of the heli, shoot it and immediately shoot the same spot when it's possible
2. shoot between the bunch of red spots
3. reload and try it again
And the last way, that worked for me:
4. Reload, shoot the Helo WITHOUT zooming in, when the runners are there, zoom in and shoot the walking spot.