Got this on King's Row on defense as Symmetra.
Next to the first objective to defend, in the direction that the enemy spawns from, is a brick wall that acts as a choke point for the first objective. Most of the enemies will funnel through here (although there is a higher-ground option as well). I set up 3 turrets on both the left and right sides of the brick wall, on the defensive side (so enemies coming through the choke point wouldn't see them, but would be hit as soon as they came through). I got maybe 5-8 kills by playing conservatively and killing random enemies that made it through with my weapon, but for the most part my turrets did the heavy lifting of, if not killing enemies outright, at least doing enough damage to get credit for the kill. Ended up with a 28 kill streak or so, and enemy never captured the objective.
Obviously still takes luck, but not a bad place/method to try if you see the opportunity. I consider myself a very average player so this one kind of fell in my lap - got this at level 22, without ever getting a kill streak over 12 before this, as a point of reference.