I recommend that you are at least level 38+ (it would still be possible from about 34-37 but in terms of not having to worry about the 30 second time requirement, I advise that you are decently above it so it will be fast and easy). Simply go to an Outpost that is in an area with the recommended level bracket including 35. I went to the 'Roman Akropolis', fast travelled to Cyrene which is in 'Kyrenaika' (North-West of the map). I used Senu to identify a guard who was level 35 (they practically all were, or higher). Make sure you have the Poison Dart Ability unlocked and then equipped, shoot the guard with the dart. Pull out your Torch and beat them to death!
I created this video to help:
Hope this helps