Despite its x0 critical percent multiplier, a successful sneak attack always results in a Sneak attack critical. Keep in mind that its critical damage bonus is 0, though, so no extra damage is actually done.
Pickpocket his shotgun ammo, shoot him once, holster your weapon and pickpocket The Kneecapper once he turns friendly again. Or leave the area and come back later, once he has unequipped it. If you're lucky you can directly pickpocket it, or after picking his ammo.
When talking Ronald into venturing for Quantums on his own (Black Widow Perk required) he usually dies on the way and his corpse can be looted near or in the Nuka-Cola Plant.
Attack him with a weak weapon (so that you don't kill him), then shoot the gun from his hands. Put the weapon away and he will stop attacking you, then simply take the gun. If he picks up the gun instead of stopping, keep shooting it out of his hands until it breaks. Take the gun, then leave and stay away for 3 days. He will be friendly again.
Using the Mr. Sandman perk allows you to kill Ronald while he is sleeping and, as well as gaining good xp, Sierra Petrovita doesn't notice he's dead. This method does induce a karma loss as the mister sandman perk does affect karma by large ammounts.