I would like to thank Cookie lMonster for bringing to our attention what is a great way of getting through this on Hard, but I would like to add a few points.
1. I would suggest going through the campaign on Easy first, not only to see how all the levels work, but to get the LEVEL 21 achievement out of the way and therefore giving you the ability to choose better weapons for the Hard run.
2. With this in mind, I used the Eagle Pistol and a Sniper Rifle. The rifle comes in useful for getting headshots, to build up your Concentration Mode quicker (for the glitch)
3. Once you've done the campaign, when you select your next character (you'll need to use all 3) just select Chapter 1 and change the Difficulty to Hard in Options. You do not need to start a New Game.
4. When doing "Cover to Cover" sections listen to the AI tell you to move up and shoot them all behind their barrier. I only ever saw 4 at any one time. Once past the barrier you are invincible again. The other way here is to use your Sniper Rifle to take them out before moving forward.
5. When driving just swerve the car left and right to take less damage. I never died on a car sequence. Again when you leave the car you are again invincible
6. Clear out all areas as you progress as sometimes you will not get a checkpoint if you just run ahead. I just Melee'd everyone to save on Ammo
7. If you have already got the Level 21 achievement, you do not need to pick anything up or do any agendas
The Hard runthrough, using Ben took me just over 3 hours.
Edit : I have just finished a Co-Op playthrough with myself as the HOST. I had it set to Easy whereas my co-op partner MrZill had it set to Hard. He got the achievement for the Hard Playthrough at the end