Whilst frustrating that all difficulties don't stack, the trade off is that you can complete all 8 levels in around 2-3 hours.
On easy difficulty, kill moves are you best friend. What I found is that when meeting an enemy, I would initially try and shoot them, presssing LT first as often it will snap you onto the enemy. If I missed and they got in cover, they tend to hide a long time, so just sprint up to them and hit A for the kill move. This way you can almost run through a lot of the levels. Good luck!