A very easy one to farm, although the achievement title isn't that clear.
When you are in close quarters combat with the silver bots (the very first enemy you meet) simply press
as they are about to hit you. If you get the timing right "GUARD PARRY" will flash up on the screen. Rinse and repeat.
You can get guard parrys against the other bots but if you're going to farm this one then there are more than enough silver bots before you meet the second enemy type.