Purchase the Death Veil ability under the assassins abilities. You only need one ability point for this
Now stealth assassinate 10 enemies like you normally would and thier bodies will disappear.
Once you purchase the ability youll probably get this naturally as in one mission you are told to be stealth and not attract any attention..
I also recommend having Shadow of Nyx and the Critical Assassination ability to make this very easy as you’ll kill most enemies with a critical assassination.