Do this.
If you have a lance-wielding cavalry (charge bonus 6+) and the enemy is set up in two or more ranks (two lines of units, not two rows of men), ride your horseys to the first rank and make sure they don't run in too deep. When they're stationary and fighting, select them and double right click the unit behind the one they're fighting.
Your cavy will begin a charge and totally obliterate whatever they have immediately in front of them.
Chaining this stuff, you can make mailed knights obliterate a whole army of general's bodyguard units.
If the enemy is set up in one line, attack it from a flank with a column-shaped cavy unit and do as above, selecting neighboring enemy units one after another.
Who said cavy charges aren't effective?