First parry the attack by holding the right mouse button and pressing space at the exact moment the enemy starts his strike. The second you duck or dodge the blade hold space whilst still holding the right mouse button. It's damn hard, but when you get it right he will *****slap the bastards
[Edited by zdeev, 4/29/2012 9:50:34 AM]
Yup. Hold RMB, press LMB when they attack to parry, and when the hero ducks or parries press Spacebar immediately to riposte.