As a long spear user, I continually hit the cavs shield even though I'm behind them and they are facing away from me (jump stabbing). It takes 5-6 stabs to kill a Cata including the first one being a full on charge, and only 3 for the plate dude on top. I've actually started using a long hafted spiked mace and jump swinging at them. 50% of the time I one shot the rider, 25% he one shots me, 25% I just get uselessly bumped for 1/5 my health.
It works better because at least that way they don't avoid me as much.