The argument was the two fighting toe to toe until death. So no kiting from the Druid. Sorry for leaving that part out.
Is the Druid really that impossible to kill?
Can an Open Hand Monk stun a fully raging Barb?
I've killed the Level 20 Moon Druid in my Runelords campaign a couple times. Once she was killed by a dragon she approached in non-wildshape form, another time was massive damage from storm & rune giants.
Level 20 Moon Druids effectively have temp hp each round equal to their Wildshape form; I can't really see how a Totem Barb could possibly beat one. But they are much more vulnerable than a level 20 Bear-Barb to a sudden massive attack.