Play mission 4, "Struck Down". You will have four attempts to use the remote sniper rifle.

Attempt 1: When you breach the door (remember to change key functions); limit of two shots.
Attempt 2: Eliminating the enemies using RPGs; limit of three to five shots.
Attempt 3: After you find Ajax and have to leave; limit of ten to fifteen shots.
Attempt 4: After you board the helicopter, defend it; limit of at least ten shots.