You want to use the cookie cutter double nado build. A high damage mainhand (high damage, not high dps) is key for Rltw damage. Get tons of armor/resists/regen/loh/ls. Gear up as though you're fighting uber ghom because uber ghom is essentially the equivalent to an elite pack with molten/firechain/etc at mp10 (at least to a ww barb). If you never die and you never have to run away, you can rack up some significant DPS over time. And these fights will take a lil time, so you want to be spamming rltw in a neat circle and keeping the pack in a tight formation.
It's all about efficiency. If you don't do enough damage, you need tons of Life on hit and Life regeneration to go with your Life steal. You want enough armor and resists to the point where you can dance around molten/arcane/rd packs as if they were trash mobs. Efficiency is key
I notice a lot of barbs neglecting their armor/ar on this forum. Sheet DPS isn't everything! You have no downtime if you never have to run from arcane sentry/molten/desecrator/electrified/fire chain/plagued. When you run away or kite, you lose TONS of dps
I'm giving this advice to POOR PEOPLE LIKE ME THAT CAN'T AFFORD TONS OF DPS. If you have high dps, than gtfo of my topic!