This can be spread across any number of games, so you should be able to achieve it through simple perseverance.
It helps to have lots of supplies when doing all those missions. Paired with the money you get from them, the only problem getting it is the time it takes. Try taking multiple missions from different empires to make the progress faster.
Pirate raids being repelled by a planet's defenses (such as an Uber Turret) count toward the achievement.