This is an alternative to the already great solutions I thought I would share.
An easy way to do this without relying on replaying DNA memories is to take a horse and ride in a circle around the Colosseum. The Colosseum is unmissable on the map. Keep following the marked road that runs a big ring around it. That road is frequented by horseback guards, civilians and walking troops. When riding, whenever you see a horseback guard, travel in the same direction as him, let him catch up to you so he is beside you and press 'X' with your hidden blade as soon as you see the Assassinate prompt on the upper right hand corner of the screen. As long as you target the horseback guards that are not near ground militia, you should assassinate and hijack them smoothly without ever becoming notorious or under threat. I did this achievement in 2 minutes, due to the sheer frequency of horse soldiers that come ride along that road. Really easy, simple and doable as soon as you are free to access the open Roman lands in the middle of the map.
As long as you dont assassinate the horseback guard near ground troops or other horseback guards, it should be a pretty effortless achievement.