I found a very easy spot to do this achievement. Around sequence 4, you'll head to Tuscany/San Gimignano. There are several "!" mission markers around. Head to the one in the southeast corner of the map. You'll be tasked with assassinating someone inside of the abbey.
There are a ton of low level enemies in brown hoodies around. What I did was assassinated the target, then got a bunch of enemies on my tail by running around the two courtyards. There are at least 15-20 enemies around the building.
They are all one hit counter kills too. Just hold RT and wait for one to attack and press X. Repeat 10 times without getting hit. If you didn't get the achievement and you only have one enemy left, let him chase you while you run around the courtyards attracting more guards.
Here's a screenshot of the location:
Any questions or comments, please leave below or PM me. Always looking for ways to make my solutions better.