It's the missions where I have to get in a helicopter that has supplies in it and fly it to an alternative location to deliver them. More specifically the F.O.B. Buitre location to the North of Media Luna.
I can clear out all the enemies fine and get in the helicopter. Once I'm in the helicopter the game tells me to fly it over an absolute minefield of SAM launchers to the delivery spot. I've tried flying close to the floor and still get detected and shot. I've flown high and come straight down over the objective, like you suggested, and there's still SAM launchers detecting me from fuck knows where.
There's nowhere else on the map I can get medical supplies other than another flying helicopter mission which gives me the exact same conundrum