This is a simple one, and you'll be able to get is pretty much at the early stages of Episode 3. What you need to do, is travel to south eastern part of the map and collect the first Insight of this chapter. It's in the Insight of Blessing location.
Next, you will need to have the poison ability from Assassin tree of abilities upgraded to level 3. With the insight found, you can enhance it to Blessing of Kronos.
Now it's just a matter of finding ten unsuspecting victims. I just visited random camps and archives and sneaked behind poor schmucks. Before stabbing them in the back, I activated the Blessing of Kronos. With Rush Assassinate, you'll get there even faster.
Don't bother trying this in a fair melee. Fifteen seconds fly past, so you'd have to find lots of enemies. Good old fashioned sneak attack works best.