I've been experimenting with a new more involved and over all more fun style of combat for Minecraft.
If Players time their blocks right before the knight swings, they can execute a parry and for 1.5 seconds the knight is vulnerable to critical hits, (3x normal damage while parried).
The Player isn't the only one who can parry, if the knight is attacked as he is putting up his shield he will parry and riposte the player.
This Mob is very dangerous if you aren't careful, but this system rewards high risk tactics.