Under the Bridges (50)
Trevor has the highest flight skill, so he is the best character choice.
You can steal a buzzard helicopter from N.O.O.S.E Headquarters (Southeast just off the city). There are also helicopters available on the police station and Los Santos hospital, although they are much larger. You are also free to use your cellphone to buy one if it is easier. Also, make sure you get a notification
when going under a bridge, and verify that your count matches the number in the corner of the video to save yourself from a headache of doing them all over. They don't always count perfectly.
*These two bridges are the hardest. Make sure you get a notification while doing them!
**The placed waypoint in the video is a tiny bit off. There are two pedestrian bridges over the highway, and you want to do the north one.
You must ensure that you get all 50 Under the Bridges as well as all 15 Knife Flights for the following achievement:
Close Shave - 20
Complete all Under the Bridge and Knife Flight challenges.