I've found that if you leave a couple of tombs incomplete, you can use those to farm kills for any weapon challenges and supernatural power kills (Though you are limited to the power at the tomb you are at).
Basically once you beat wave 3 of a tomb, instead of going back to the tomb to finish, you can stay in the area and get kills on what stays as a level 2 notoriety, which means you'll get an endless spawn of regular enemies to play around with.
When you're done farming kills for the session, you can always just leave the area or complete the tomb mission if you have more still available.