Since most weapons do barely any damage on leg hits I trained myself to try it only with the silver PP7 or the moonraker laser - especially helpful if there is a door or some glass between you and your victim.
Any other attempts are bound to fail since even new players knowing nothing about the crosshair will eventually kill you sooner or later while you're trying to shoot their legs only - or they'll be closing in on you because they don't hit you making you shoot their waist by accident.
That stated: yes, it's kinda tedious and I've got Scaramanga's Pride along with Shaken, But Not Disturbed (00 Agent) in a MWGG match way before Break A Leg (I'm actually not even sure if I've got it yet).
I propose: having other conditions like "just hitting" a leg but since it's "payback time" playing Valentin shooting Bond. Would make it easier because it's just to hit but at the same time it's a little more tricky (not as tricky as now, of course) by having additional conditions.
EDIT:
It's sad the WNx Page show only those achievement tracked by their own servers and after Beta 4. Since I've got most stuff before Beta 4 and Scaramanga's Pride and Shaken, But Not Disturbed on another server it makes me sad I'd have to do these all over again if I wanted them to show up on the achievement tab. (I guess these two are the most difficult to obtain along with Domino Effect)