Finally finished the Gun Runners Arsenal DLC. I just needed the Pros Only challenges. I killed 15 named NCR Rangers and commissioned officer with a chainsaw (that's right!) which wasn't that difficult - it was more difficult to actually find them. Then I killed 5 adult Deathclaws with a silenced .22 Pistol - they didn't know what hit them - again and again and again... And last but not least I destroyed 10 Mojave Wasteland Mr. Gutsys and Sentry Bots with Brass Knuckles - Man, are my arms tired..