Just a minor spoiler for this one, but it is still a spoiler so be careful.
'The Outsider' is level 20 and he patrols around Alexandria. Now I was level 24 when I did this, but I think you could manage it at 17 with a bit of luck and grit.
He has a sword and shield so I would recommend having a weapon with a large swing. Personally I used a heavy two handed axe. Then just wail on him with the
to break his shield, and use
to dodge around him.
After you defeat him you must confirm the kill otherwise it will not count, so go over to him and hold
so he doesn't get back up. After this the achievement should pop.
Let's keep cheevin' together!