Posted by: coolcor13
If you've beaten the level, you can go back and play it whenever you want. You might have to start from the beginning or a Rally Point if you didn't save, but the level is yours.
I mean to get Completed Legendary achievement do you have to finish them all on Legendary in a row until you finish and not quit, play every level in a row and not start a new campaign(and your able to play other games when you don't want to play or your friends don't want to play H3) or can you just choose any level in any order and beat it on Legendary until all say when you pick a level "Coop: Legendary(symbol)" and still get the Achievement.