Kill 10 guards with Arrow Storm

First and foremost, get yourself noticed by the guards, tackling works just fine. I also recommend you search for a section of the map that has a lot of guards in one area, so that making one set chase you, will alert the others. (Good spot early on is near Castel Sant' Angelo) Arrow Storm can only be done once you have at least 6 assassins recruited in chapter 4. This video does not show how to round up guards as that should be self explanatory.