I found that knights charging your line headon are going to smash through, or make a serious dent, no matter what you have in front of them. But once you stall the knights and counter attack them from the flank with polearms, they die fast. The AI will do this to your knights too so you HAVE to break off a change once it hits home then wheel and charge again, unless the AI has redeployed, which it usually does.