Should anyone still be trying for this one, I found another easy way of doing this. At the end of the second proper level in the Instincts campaign, called 'native', you need to cover Val with a sniper rifle. This is right before you get captured by Crowe's men. To get to the sniper position you need to climb up some vines and you'll end up behind a sniper preoccupied with shooting Val. The game obviously prompts you to backstab him but what's wonderful about this is that you get a checkpoint right as you end up behind the sniper. Stab him in the back => reload checkpoint => proceed to do it 40 times.
If you downvote, please tell me why, so I can improve the solution.