Fast rolling with weapons that change damage vs multiwound units

Hi. Specifically my question is how one handles making attacks with weapons that can change damage mid attack process vs multiple, multi-wound enemies. I don't mean variable damage weapons (d3, d6, etc), but like Incubi swinging against a unit of 3 wound models, and some of their attacks become higher damage from their to wound rolls.

Does anyone have a recommendation on how those wounds are then allocated and saved against fairly? With variable damage weapons I've played it as rolling the damage one at a time after all saves are made, but since the damage changes for Incubi two steps earlier, there are likely to be a lot more dice to try and do that with. Can the attacking player ask to have all of one type saved against first? Or is the defending player allowed to say, allocate a 2 damage and then a 4 damage attack on a 3 wound model? Or is the most fair option to slow that fight to a crawl with one wound roll at a time?

I am aware that yes, the technical procedure is a single attack rolled with a single die at a time. No one I know plays that way with EVERY attack and save during a full game, so I was curious what some of you either do or would suggest in the event this comes up during a game (and matters, like the above theoretical example)

Thank you!

Rule 221 of the core rulebook clarifies fast rolling.
The rule is that fast rolling requires all attacks to have the same Ballistic/Weapon Skill, Strength, Armour Penetration, Damage, Abilities, and must all be directed at the same unit.

Since all incubi klaive attacks are S5 AP-3 Damage 2, and all of them are covered by the Lethal Precision and Blade Artists abilities, you are free to fast-roll incubi attacks (and the attacks of any/every other drukhari model, for that matter - since we get Blade Artists on almost everything it'd be quite a pain if we couldn't fast roll!).

The defender - not the attacker - then allocates the attacks, one at a time, making saves and taking damage as appropriate.

So if you have a trio of incubi attacking a unit of bike marines, and you score two 6s to wound and two regular wounds, the defending player is free to try and "bait" the high-damage attacks by rolling his saves against the normal wounds until he fails one, then rolling the D3 AP-4 ones so a bike marine with 1 wound left can eat the superpowered hit - so even if he fails all saves he'd still lose two models instead of three.

You just divide the rolls at the point they change.

For example, any weapon that increases AP on a 6 to wound.
10 rolls to hit, 10 wounds, but four have extra AP now.
Opponent then takes the pool of wounds, and allocates as normal, knowing that 4 have extra AP oveer the others.

As with Incubi.
10 hits, 8 wounds, 2 doing extra damage.
Opponent allocates, knowing that as they do some do more damage than the others. Then they make their saves.

It's an interesting question, the term "fairly" is important here, and is I suppose somewhat subjective.

Technically, the defender is free to allocate the the damage as they see fit, but then again you are also within your rights to refuse to fast roll, and thereby slow the game down to a crawl. You are doing them a favour by fast rolling, and IMO, the defender attempting to 'waste' your damage is poor gamesmanship, not a valid tactical gambit.

What works for me is that I will agree with the other player before the game starts whether all damage will be saved against highest to lowest, or the other was around. And it will carry on that way for the whole game.