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....

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)...