FFX-2: The Gunner Achievement
FFX-2: The Gunner15
Reach the highest level (LVMAX) in Gunner's Gauntlet
Gunner's Gauntlet is a mini-game found in Besaid Village in Chapters 2, 3, and 5. The goal is to shoot as many fiends as possible as you make your way to the beach, and you want to score 2800 points to obtain this achievement. You can chain kills together and increase your score multiplier, while getting hit will reset the multiplier. There is a spot in the course where enemies will ignore you and run right on by, allowing you to rack up all 2800 points with no trouble, after which you can simple run to the end to complete the run and unlock the achievement. The purple/yellow creature is important to kill as it yields Volley ammo which will kill every enemy on screen at once. You can also create this by combining two Death ammo. See the video below for this method.
Note: You'll have to complete the game a few times to get to at least Level 6 before you'll have enough enemies coming through to make it to 2800 points. Keep playing and getting as high of a score as possible to rank up the game to where you'll be successful for the achievement.