Given assize is probably needed, it is true that a solo healer needs to stay with the party.
But the party should adjust by not having the WHM leave stack, and you don't need to heal the away flares immediately since there's no other raidwides for a bit after Unholy (6 people in stack is survivable at 80k hp plus change per person).
I think it's possible, but definitely harder than the shield jobs.
