When I'm playing as defense then I just walk around with the crowd while being nearby our artifact.
This tactic is way more effective than camping on rooftops or specific spots to get areial kills, in my opinion.
1. They usually don't see me coming, and when they do, it's already to late for them.
This way I don't even have to waste an ability to kill someone.
2. If you're camping on a specific spot and don't manage to kill that thief from your spot, then you're pretty much ****ed in 8/10.
Even that one spot from where you can kill any player in 90% of the artifact circle is not effective at all, just throw a smokebomb on the player who's camping there, or even better, throw a knife at him: problem solved.
Btw. thanks for your input so far, guys.