Hi there! this is my guide to gaining the Close Shave Trophy, these videos have not been made by me, these videos are by XCVii007r1 who shows the best way of gaining this trophy. The 1st video is under the bridge, the best aircraft to use on this I found was the buzzard chopper (I am not a very good pilot!) also use Trevor as he has more flying skill than Michael and Franklin.
The 2nd video is for the Knife Flight walkthrough. I found again using Trevor due to his better flying abilities than Michael and Franklin, also use the duster as it is slow enough for the job at hand. I'd also like to quickly mention as you are going along doing the under the bridge and knife flights, I'd recommend using the quick save feature on the games phone, just incase anything was to happen. Any problems send me a message I am more than happy to help :)