Actually, If I remember correctly, the Lord Marshall's speed induces not an after-image - but a before-image. Wave your hand in front of your face - a shape of your hand is cast by light and stays in place where your hand was for a split second or two, creating an image of where your hand used to be. The Lord Marshall's speed works backwards - instead of moving in a burst of speed and leaving a temporary image behind, he actually casts a before-image (a shape of where he's going to be) and then moves in a burst of speed. You can imagine how unbelievably dense this is to use, and how unbelievably easy it would be for someone to counter. I think the first few times the Lord Marshall used his speed, his opponants would choke from surprise and falter, leading to their death. However, with his speed casting an image of where he was going to be, to anyone with simple 20-20 Vision, it is very easy to counter and not surprising in the least that Riddick was able to defend against it. Gurney Halleck could likely kill the Lord Marshall, in his sleep. Now in terms of Riddick versus Paul, compared to Paul, or a host of other Dune fighters, Riddick is a one-dimensional fighter. Riddick has strength and a bit of durability, but doesn't have the speed Paul has, nor the precision, nor the training in a martial art that is likely more advanced than anything Riddick has ever known. Duncan Idaho or Gurney Halleck would carve Riddick open like a Thanksgiving Day turkey, with perhaps, moderate difficulty. Against Paul Atreides? Riddick gets beaten badly, and easily. Paul wins.