Do To Ruam's 75% resistance while charging, stacked on top of his potential 4000 soul armor from soul harvest, and 750 additional max health (from sinister allies) implies ruam can stay at a consistent 9250 to 16,187.5 hp... Sooo, Stun him ALOT....

Don't just mindlessly CC him. Once it's over or he walks out of the area, he can just Juggernaut away. Wait until the animation starts and then CC him to shut it down completely.

Also other then just stunning him a lot to avoid him being unkillable, play high mobility characters and be unpredictable (long range sucks when he can just charge you sooo avoid squishy snipers unless you have mobility like kinesa with transport)...