(Note: This can be obtained through multiple playthroughs. Not required to get all 1000 with one run.)
What I've been doing is wherever there is a Fade Rift, I'll defeat a few of the demons and then leave the battle area (Just keep backing away from the battle. You'll see the enemy despawn, then just walk towards it again).
Doing this resets the spawn of the rift, so that the same demons that you just defeated respawn.
So you can just do this a few times on every Fade Rift or repeatedly on the same one until you decide to stop.
In theory, you could get all 1000 kills on the first rift in the Hinterlands.
There's my guide to it! Please explain any negative votes and hope this helps!