To do a perfect lightsaber deflection, first, you must obviously be using a character that uses lightsabers, and second, need to tap X as a blaster bolt is traveling towards your character. If done right, the blaster bolt should be deflected straight back at the person who fired it. The timing on the press of X is very generous. Usually after any point that the bolt was fired you can press it and it will deflect. The only exception is when the bolt was fired at extreme ranges.
Rinse and repeat 199 more times and the achievement is yours.
Unlike what some others have suggested, this is ability does not require the use of the left stick at all.
Holding X does not produce a perfect deflection, and therefore doesn't count to the achievement, though it does at least deflect the bolt to the side.