If you are concerned about this achievement and you are just now starting the game, don't worry! There is an easy difficulty work around "glitch". But in order to do this you must be on a completely fresh playthrough, or be in a friends fresh playthrough. "Never completed the mission"
Simply start up the mission on recruit, play until you reach the final checkpoint of that mission. Which if you don't know the checkpoints i found this site to be off help.
Link to help: https://guides.cheatcc.com/call-of-duty-black-ops-iii-guide-
Once you make it to the last checkpoint, save and quit your game, then once you are loaded back into the main lobby where you see resume game change the difficulty from recruit to realistic and just hit resume game and finish the mission. Do this for each mission and boom done. Hope this help, I know i wish i knew about this when i first did the game.
Credit: This was found in a PlayStation Trophy Community, then IamMetallica121 and myself tested it out and it works perfectly.