This achievement is for the police chase part of the Weird Science mission, it begins at the part when Jimmy picks you up with his car.
You get infinite RPG shots and there is probably 100 or so total targets during the chase so you get plenty of opportunity. Don’t forget you can lock onto tougher to aim at military/police vehicles with
Helicopters, police cars, military suvs all should count towards the 45 needed.
Worst case scenario if you do mess up, try to make a save before you start the mission, just in case. This way you could always just try again if you miss it.