This achievement is, as the name suggests, awarded for completing all of the Campaign on either Hardcore or Insane difficulty.
The achievement is stackable, so if you complete on Insane you'll automatically unlock any lower-level difficulty completion achievements.
If you're struggling to complete the campaign on your own on either of these difficulties, there is a well-known and well-publicised solution where you can play co-op with someone and have them play on Casual difficulty and do all the work, while you spend time in cover and hiding.
There are a few sections of the campaign where you get separated and you'll have to do some of the work
, but the player on Casual can assist you by laying down cover fire or killing your opponents from their side.
The hardest section for this method if you're on Insane is the motorbike escape at the end of Chapter 2, but the Casual player can still do most of the work. Failing this, just keep working through it a Checkpoint at a time!