Bushido
With it and the Book of Bushido (looted from Ronin or buy from player vendors), the Samurai can perform a variety of special abilities that mostly are defensive in nature - but can be used to quickly and honorably defeat the toughest of opponents.
Training:
0-30: - Buy from NPC Samurai at Tokuno Zento
30-60: - Cast Confidence
60-75: - Cast Counter Attack
75-105: - Cast Evasion
105-120: - Cast Momentum Strike
The best training method I've found is to train the skill up from an NPC and use jewelry to get yourself to 50. Then use Lightning Strike on your opponents until you reach a real skill of 50; remove your jewelry and continue to use Lightning Strike until you reach 110. At this point switch to Momentum Strike. Lighting Strike will, however, take you all the way to 120 if you prefer.