i got this achievement second time playing through and a little experimenting
first, you obviously need the chain attacks skill
after that i suggest you find somewhere nice and secluded, but a pretty good open space as you're character is going to move forward while attacking, and if you try attacking a wall or a tree or something "unbreakable" then it will stop your attack and your efforts are wasted
as for accomplishing it, you start your chain attacking after your first NORMAL attack, i've heard other people saying its done by flourishes, but its not, the entire chain (or at least the easiest way) is to do it by swinging normally.
what you want to do is just as your sword gets as far back as its going to go on the backswing, you hit X again. when timed properly you will see a blue trail behind your sword. then you hit X again just at the end of that backswing, and keep doing that until the achievement unlocks.
be warned, its going to slightly increase in speed with each swing, so there's no countdown or anything
