Note that the skill of Bushido is related to this one, and both tend to complement each other. A high enough Parrying skill enables the player to use special moves.
Actually training this skill is difficult. It can only be achived by actively using it in combat, which especially at the beginning will be the cause of much injury. Therefore to quickly advance it, one has to get the best protection, before literally battling masses of foes, and running away once things go critical. During battle it is important to actually get hit a lot, therefore masses of weak opponents are the best choice.