The quickest way to get this one (and also the most fun) is to just go to a city you like (I used Damascus) and make a mess, so the guards will pursue you.
Then don't escape and fight them, one by one, with either the sword or the knife in close combat.
The thing is that you have to tap "X" when you hit the enemy's sword (you will see a flash/clash), then just when you see that, tap "X" again, so Altair will continue attacking.
after 2 or 3 clashes you will see an animation depending on the weapon you were using (there are like 7 or 8 animations) that will kill the enemy.
For example with the knife you can see an animation that chains a combo of 2 punches and then cuts his throat with the knife.
Repeat with 49 more slodiers and the achievement will pop up.
Just keep in mind the timing (you will need some soldiers to get it)