When you killed a shadow grim under the lamp in the middle of the room it was suppose to spawn another grim of either fire, or ice under the light. Since this mechanic was broken, you had to spawn multiple grims as you did in the video, but the AoE damage from the grims was too much without bringing another tank, or more healers. By bringing the extra healer, or tank, you lost too much dps for this fight to be possible at level 75.
P.S. this fight was completely impossible before they nerfed the grims from having 90k morale to 35k morale.
P.S.S. saruman had a similar problem until they removed the polarity from the fight instead of just making it a frontal as intended.