This achievement can be done in the very 1st fight you get into as long as you are calm.
For the first guy you can just spam
Once he's down the first thing the next guy will do is swing his knife but in slow motion so press
once then tap
twice to hit him.
Now wait for him to swing and again press
once and tap
twice, This is where you need to be quick his next swing will be almost instant so you need to be quick on the
button. rince and reapeat until dead, he should only need a couple more hits.
Once he is down the first thing you need to press is
which should make you lunge toward the 3rd guy to break his guard then tap
twice. rinse and repeat this until dead.
If done right you should end the fight with over 40 combo. see video for reference.