This game requires that you fly at a speed of 2463 km/h during a story mission. Due to a glitch you cannot reach mach 2 using imperial units so you must have your units set to metric. This trophy also cannot be done during replay story mission mode, it must be done while doing the main campaign.
The best mission to grind this trophy is in mission two. Just fly as fast as possible around the map turning the plane using L1/R1 instead of the analog sticks to maintain speed. It helps to obtain the speed increase perk from the PEC rewards. You can quit the mission at anytime and it will save your progress so you can return to it later if necessary.