Cutting off limbs isn't terribly difficult - though you will need an edged weapon, machete or sickle seems to work very well - but this is so much easier when they can't fight back! Just kick the crap out of them and once they're on the ground aim at the arms or legs and off they fly.
Like most of the Zed smashing achievements, this one is stackable between characters so if you're finding it difficult later in the game you can start a new character and keep going.