So, this is basically another Arch-Moon-Druid issue, where you need a spell to put someone under instead of killing them through hp damage. Doable at this level, though something to keep in mind when encounter building, which is annoying admittedly. But its worth noting that a simple Sleep spell at this point (or Eyebite, or similar) will knock a barbarian out of rage and kill it at this level.
Or just kill the rest of the group? Party wipe with one survivor is pretty much end game anyways, usually.
Barbarians are good at being tanks. Its kind of their thing. So, instead of doing a face-forwards pure damage brawl (which is going to be difficult in the first place), you'll need a bit more tactics.