Do NOT utilise quick resume for this game as it seems doing so somehow glitches the achievement out!! Completely quit out and go back into it!
So this is a relatively simple achievement, and I say relatively because it would be potentially tricky to do with a human enemy.
So there are 2 points in the game you practise assassinating training dummys. One is early on in Fornburg when Basim is "Showing you the way of the assassins".
The other is when you get to England and set up your settlement...
What you need to do here is unlock "The Hidden Ones" guild and you will have a chat with Hysham and a small mission (Learn how to perform Leap Of Faith).
After this, the dummys appear.
So call your horse by holding the
Jump on using
When you are next to a target, press
until its health bar has a small fraction to go.
to go on top of your horse.
Then, as long as you get the "Assassinate
" prompt showing.. by all means press RB and the achievement should unlock.
Hope this helps!