Instructions
Press the "X" button three times to do three weapon attacks. Time each attack so that you press the "X" button at the end of each weapon swing. The first two weapon swings will be normal attacks, but a blue trail appears behind your weapon after the third swing. This is the key to successfully doing a chain attack.
Swing your weapon three times again, timing each attack as you did in Step 1. On the third swing, press the "X" button once the weapon, followed by the blue trail, reaches the center of the swing. This should continue the chain attack. Time the button push on the fourth swing just as you did on the third swing. Continue in this manner to chain more attacks together. Each consecutive attack will also increase the weapon swing speed.
Keep a chain attack going for five or more consecutive attacks to unlock "The Duelist" achievement. It is possible to chain 20 or more attacks together in this fashion with proper timing.