Hi,
I have a little experience with Deamonic battle in WFB3 : 6000 vs 6000
Even if the champions were already created, this battle required several hours of preparation. Chaos attributes must be drawn, profiles changed accordingly, and the many spells and profiles (the Jabberwock, here) still need to be drawn. I wrote two A4 rectos (written small) for each army : a page for demons and an other for mortals. Predicting the difficulty of drawing spells during battle, I had made a list of one hundred level 1 spells prepped for lesser deamons and another one of about twenty spells for Great Deamons. I used a generator of dice available on the web (it's faster than a hand). In the course of battle, there is still much written: the Demonic Power Points, Magic Points, active spells, etc ...
The battle itself was quite pleasant. Since it is impossible to retain all the profiles - modified by the attributes - and the special rules, we spent a moment in books and papers. The most painful point is the management of minor demon spells. The "small" spells that they throw accumulate on them beyond reasonable: "Hands of Fire", "Hand of Death", "Strength of Combat", without mentioning the invocations (hero 5 skeleton, familiar, ..). It's a lot ! As I have assumed, the most useful spells are not the most impressive: the small spells of improvement give good bonuses and can not be canceled, it is also the fate of "Debility" (Battle level 1) which neutralized the Lord of Change once he had exhausted his DPP. In the meantime, lots of projectiles ("Fire Balls", "assault of stones", ...) multiply the minor demons to prematurely toast their DPP because they can be devastating on their Endurances of 3.
Finally, even if he has things well, I do not think WFB3 is the best version to play demonic battles. It's too heavy to handle... I think WFB4 or 5 should do better with a few adaptations.
I did another test where I used only the powers of the Rogue Trader. It simplifies the tables and gives more usable powers (A lot of spells of battle magic are of no very useful).