For this achievement you will need to at least one gold medal in
Ammunition shooting range
A triathalon
A street race
A off-road race
A sea race
Flight school lesson
For the shooting range the railgun challenges are the easiest to get gold in (find this under the special tab)
For the triathalon if you have maximum stamina it’s easy to get gold in any of them but I recommend the Vespucci triathalon for the gold medal.
For the street race any of them will do as they are all quite easy if you are using Franklin but I recommend the Hawick race as it is the first one unlocked after the strangers and freaks mission
For the off road race use Franklin as he has the special ability for driving. They are all very easy so it shouldn’t be much hassle for any of them
For the sea race any one will do. The races are very easy.
For the flight school the first lesson is very easy as all you have to do is take off. Use Trevor for this as he is the character that has the most flying skill to start off with.