You are supposed to
group monsters with Furious Charge + Ancient Spear;
keep up party buffs - Ignore Pain and War Cry;
help with DPS using Ground Stomp (as DPS classes can use Bane of Trapped or other debuff dependent items) and triggering your Oculus Ring buff.
Comes from your build Skills and Runes page:
The Support Barb ...
According to this thread:
It is not possible to acquire it. This is due to the fact that 360 and PS3 versions are limited to version 2.0.6 of the game. The Cube was added in patch 2.3 and other platforms are already on patch 2.6.7.
So any version lower than 2.3 won't have Kanai's Cube.