For this achievement you must obtan an AAA ranking on 24 missions. My best advice would be to ensure you have the Dark Booster and Ulti-Mondo costumes and the game's difficulty is set to Easy Mode. Refer to any number of YouTube videos and forum threads for the specific issues you are having. Most notably the Bike Sub-Mission is the most notorious for giving gamers an issue when it comes to AAA ranking.
Note: Some players have reported that the achievement has not popped unless they've gained all rankings on one difficulty, others have reported that they've received the achievement despite having the rankings scattered across multiple difficulties.