NOTE: In order to be ranked on the World Ranking leaderboard you must be playing on "Hard" difficulty.
In order to check the ranking on any mission you've played on, go to that specific mission. When the mission is opened up you may press "Y" to go to World Ranking (it'll give you rankings based on Global, Nearby, and Friends).
As soon as your rankings load up, back out to the globe screen and your achievement should pop immediately afterward.
Happy hunting!