If like me you maxed out your skills before getting this achievement, i found a way so that Lara always incapacitates before killing the enemies.
I would stand still and wait for a melee enemies to rush me, then dodge when the animation of them swinging their weapon starts (B without a direction, its not needed) then the quick time event appears, if you can press y as soon as the two circles meet and i do mean as soon as she will stick an arrow in their knee or incapacitate them.
if you let the QTE go on for too long Lara will go for the insta-kill move.
p.s you can make bow wielding enemies use melee if you rush them, helps to get more melee enemies further in the game.
hope this helps!