This will be your last and final achievement. Unlocks after all other 48 achievements have been unlocked.
Some have reported this as a glitched achievement, and have had problems unlocking this achievement. It is not 100% confirmed on how this will glitch on you, but people say it has to do with the multiplayer.
1) While in multiplayer, make sure to finish the entire match and not quit after you have unlocked your achievement. Example, if your going for "Mr. Unpopular" Pick Meg in multiplayer, after you unlock the achievement, DO NOT quit, finish of the match so your progress saves.
2) Story Mode - Secondary Objectives - Again, similar like tip 1. When you complete a secondary object in a level, DO NOT quit. Continue on with the level until you complete it. Also, if you replay a level that you are missing a collectible in, the counter will restart and you will have to collect all the collectables for that level again.
3) If you replay any level to farm for money or kills. You must collect all secondary objectives again for that specific level and finish it.