We have 2 very good theorys here.
One being the Knockback from Yogg causing the mobs to evade. Is this possible? There may also be somewhere you can wall jump up, or something similiar and cause the mobs to evade bug with let's say, healing aggro.
The second theory is leaving a healer in the Brain Room with DI. You will also need something to heal effectively, so let's pick a warlock for that.
At the upper left corner, there is a spot where the part of the raid outside gain BL when the Portal Team uses it in the Brain Room. This doesn't occur at the other parts of the room from what I've experienced. What if the raid stands there and just taunts the mobs to the spot and then let the left-inside healer get healing aggro by healing the lock? This should cause the mobs to evade.