Archaon, Triple-headed Monstrosity - Damage allocation question

[sorokyl]

Core rules say 
ALLOCATING WOUNDS
Once all of a unit’s attacks have been resolved, add up the damage that was inflicted. The player commanding the target unit must then allocate a number of wounds to the target unit equal to the damage that was inflicted.

Archaon - Triple-headed Monstrosity says: 
After attacking with Droghar's Three Heads you can pick one of the following effects if at least one model was slain by those attacks

The warscroll seems to indicate the model must be slain by that particular weapon, but if all attacks happen simultaneously (you do not allocate any wounds until all attacks are made), the wounds are in a common pool, and how do you know which wound "slayed" the model? 

I found this in @Nicos "London Calling 2017" FAQ , is this still relevant and is this how other people view this:

 

Slaying heroes/monsters with special weapons

Q: How do weapons which have a special rule if they "kill" an enemy hero/monster work given that the opponent has the right to allocate damage from your unit when you attack with multiple weapons (e.g. Mortarch of Blood, Archaon's Triple Headed Monstrosity and Gordrakk's Strength from Victory).

A: To avoid the opponent rigging this (choosing damage to another weapon as the final wound to allocate - assuming that this is even possible) or you having to roll attacks one at a time, if the weapon in question contributes any damage to the target hero/monster which then dies from the attacking unit's melee attacks (all in the course of the same pile in and attack), then the buff triggers. For Gordrakk, you can pick which of Smasha or Kunnin' gets the buff (not both weapons from a single slain hero).

[FRoper]

i would simply play it as when rolling, roll each head separately and then simply when rolling damage saves, roll for each head, then you can simply allocate damage and if a head does kill you can choose.