It's been FAQ'd. 19.4
Q: Some effects inflict damage that can only be allocated to Wizards. Can you explain how such damage is allocated to a unit that is not a Wizard but includes models that are Wizards, e.g. a High Warden from a unit of Vanari Auralan Wardens?
A: The wounds are allocated to the unit as normal (core rules, 14.1)
So it's the third option. The unit would take 6 wounds however they can be allocated to any models in the unit