When you get to Sequence 8 - Hunted, right at the start you will have to go to the right and unlock a hiding spot by pushing forward on
and then some guards will run by, as soon as they pass run to the next open door hiding spot (there is just enough time to get there before the guards see you. Now this time, as the guard run by you want to press
to assassinate one of them, it is best to aim to assassinate the second one as sometimes when you assassinate the first one, the second one will see you.
Repeat this as many times as you need to as it counts for hiding a body, and it will also go towards this achievement if you don't already have it.
I have attached a video to help.