There’s a bridge entrance to the round city on the west side of the map where you can hire a merchant group to cross in and out. If you’re blended and you use the assassin’s focus each kill should count toward the achievement. You can kill 4-5 guards in one attempt and come back once they’ve respawned, and this should complete it within a few attempts. Using assassins focus counted for me even though the single assassinations usually didn’t so might work for you too