To unlock this achievement you need reach 500,000 XP. You can gain XP in several ways including:

Completing missions
Completing challenges
Completing jobs
Finding Aerocaches

Both Missions and jobs have XP for completing them as well as bonus XP on offer throughout the duration of the task. For instance, most jobs will reward bonus XP for flying them at night. For information on finding Aerocaches see "Aerocache Hound". You should gain this achievement with natural progression throughout the game. However, if you do not, the fastest way of gaining extra XP is to complete one of the earlier landing challenges like Stearman Landing challenge 1 over and over.

~Int