Having played on a horse for more than one year on mm and since the beginning of NW, I can say you that, in fact, any well trained infantry player can kill the greatest horseman easily if their fight face to face. So the best way to attack, each cavalry class included, is to attack right in the back of a line.
It's one of the officer's works to look around wile he gives orders. During a mêlée including allies and enemies, trying to avoid at most teamkill, the best way is to hit an enemy to knock him out and to give the ally fighter an opportunity to kill him and go help the others.
You should be happy that being hit by a horse at full charge speed doesn't kill you in oneshoot, cause it's what should happen if we were on a real battlefield