This was the last thing I had to do to get Reckoner and I agree that it’s a real pain. I think that they are bugged and don’t pop up correctly when you meet the 12 kill in 3 second requirement or at least they were at one time. Dunno if it was ever fixed or not. Personally I had the most success using Riskrunner to spray down a fresh group of adds before my teammates could get over to them and just praying the medal would show up. Did that over and over til I got what I needed. Good luck.