Worst case scenario, it's rendered unattainable due to a glitch, or requires an online connection to a server that no longer exists, never did exist, or no longer ever has enough people on it at once to satisfy the achievement. You may have to beat the game on Harder Than Hard or under conditions that would otherwise be a Self-Imposed Challenge. It might require you to clear the Brutal Bonus Level or defeat the nastiest Bonus Boss. Maybe it's impossible or nearly so without buying something with real money first. Often it's something absolutely no one would think to do. It often needs to be unlocked in That One Level, during That One Sidequest (of which it is a Sub-Trope), or while fighting That One Boss. It can easily involve The Computer Is a Cheating Bastard, Luck-Based Mission, Fake Longevity, or Fake Difficulty, though in many cases, the difficulty is indeed legit. Of course, there are those who are apathetic about it, but they are unlikely to be hunting for these in the first place. Others delight in actually getting these. Some players loathe these with a passion. It's particularly jarring to players when most of them were easy, or at least fairly easy, to obtain. Related to Last Lousy Point, this is that one achievement (or trophy) that keeps you from true 100% Completion.

Airborne kingdom trophy guide