To do this achievement you have to do this in ranked match/duel which is not specified at first but the description says after getting it 'Break your opponents arm and leg by grappling before final stiking them in a ranked duel', just do as it says and break their arm(grapple+X) and leg(grapple+A) in grapple mode then final strike/exicute them(grapple+Y) and the achievement pops up.
Hope this helps
o0 JdogJ 0o