What brennan said. That's pretty much the most reliable thing. Unfortunately it doesn't work well if you're impatient. You could also try for headshots from afar, but they'd have to be lured into the open or something so that they don't escape.
Quite simply: RD users make their game by using Ammo Salvage, which really only takes place when they kill someone. Therefore, the most important thing you can do is Stay Alive. I suggest going in like you normally do, and THE SECOND you see the Rad go down, get out of there, either luring him away from the rad, or getting out so you can regen your health before going back in to fight him after he's out of rads.
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My favorite is tpc mines. RD users like to run up on you and slam a grenade at you. Set up mines at a choke point that they cannot see and bait them in with your red dot. Of course if you like the scar you will have to run armory.