x.x You'd have to be so lucky, or so smart, that it's basically [i]im[/i]possible. For one thing, they're never going to be alone, especially not if they're guarding someone or something... 

Obviously, the best overall tactics for the resistance are going to involve hit-and-run attacks, the avoidance of Knights and Elite Guards altogether, lots of explosives, key assassinations, incitement of mass uprising and lots of fantabulous SCIENCE!, but we can leave those discussions to the IC.

EDIT: What we need, in all honesty, is to capture some Anti-Materiel rifles, or find some out in the Wasteland. Those are just about the only thing (apart from EMP weapons and high explosives) that would be effective against armor that powerful.