To complete this the fastest way possible choose an easy scenario, save right before you're about to win the scenario. Load and win 5 times on the same scenario. On the 5th win you should get the achievement.
If you're achievement hunting I suggest winning all three Mass Transit DLC scenarios first and performing the above two more time on one of the Mass Transit scenarios. Floodland is a good 4th option as it unlocks a unique building but isn't required for any achievements. If you complete all the Natural Disaster DLC scenarios you'll unlock its monument, again not required for an achievement.