Not so much a solution but a tip. If you're like me and you have this game on multiple platforms and you completed all of the challenges on one system and unlock the achievement but when going to the other system to sync your save and you find that the achievement will not unlock.
My first thought was to just play a level and complete it and it should unlock but that didn't work. So the solution to ungltiching your achievement is to complete a weekly challenge which will unlock this achievement and also the "A for Effort" achievement which is to complete all operation F Challenges.
Hope this helps for those that were like me!
You can still do this in 2019, if you set your date on the PC back to 2014. The weekly challenge will appear and the achievements (Extra Credit and A For Effort) will pop after you finish just one of the challenges.
Thanks to jaKopoLeon for that info!