The ways to contribute that I listed are all things that we've done already in previous battles and conflicts. Assassinating the Warbosses in Karagril, causing chaos in Karag Lhune, the Kurgan battlecamp, etc.
It's more efficient to leverage the capabilities that Mathilde has that are unique to her, of which there are one, instead of using her as another standard Battle Wizard, of which there are dozens (or hundreds, across all the colleges).
IIRC there is less then a hundred battle wizards total. Even after adding ~ 50 wizard lords that is not hundreds. As for the things we did, none of them happened in a middle of a major battle, with tens if not hundreds of thousands of combatants on each side. And even if we want to lean into our unique skills killy BM can still be insanely useful, see hypothetical below.
Full on army vs army. Mathilde can historically rely on having magic support on her side, because this sort of thing does not happen suddenly, and Mathilde always makes sure to recruit some mages. So being MMM dispenser, counterspeller or scout are better uses of her time. Some killy magic could have uses, but it's pretty marginal here.
Everchosen's army can be expected to have plenty of casters of their own. It is entirely possible and even likely that in such a battle every spellcaster on Imperial side would have to focus on casting/dispelling just to prevent the army from being nuked/debuffed to oblivion.
If it's an Everchosen Showed Up, All Hands On Deck Situation than any battle mathilde is participating will ALREADY HAVE the imperial doctrinal answer to greater daemons: massed cannonfire. If they don't the battle is probably already lost and mathilde should probably be helping important notables escape.
There are plenty of reasons canon may not be available. It could they don't have LoS. It could be they are preoccupied with some other target. It could be they are disrupted by enemy's lighter elements (furies, light cav etc). In general I do not find the argument of "the others can do it" to be a good one to avoid learning magic with an Everchosen on the horizon.
Now a lot of this is subjective, but by my judgement most of the scenarios where casing battle magic we don't already know is the "best" thing for us to be doing are rather involved. Furthermore with all the other great options we have (dispelling, MMM, dragonflask, sword, scouting, etc) even where BM is better then our other options, it's not that much better.
I have a hypothetical about that, see below.
It is extremely useful in warfare and cannot be substituted by mass cannon fire, but that is entirely a separate matter from the discussed "a great daemon showed up" where it very much can be substituted by massed cannon fire.
No, the matter being discussed is "the Everchosen's army shows up and we need to fight it".
Because it is just as true for their melee heroes obviously, the human sized ones at least of which he is bound to have many. This goes back to the crux of the argument of Mathilde's skillset not being magical artillery of us going out of our way not to be magical artillery all quest, of Regimand warning us not to learn standard battle magic beyond Smoke and Mirrors. This is what I do not understand about the battle magic argument, it comes to these desperate situations where we are the only wizard in an army or the emperor orders us to fight with spells because 'all hands on deck' or we are facing a Greater Daemon who is about to eat Panoramia alone and then concludes that 1 AP spent on battle magic and one cast of Pit of Shades will solve everything... well darn why did we bother all that much with all those other skills then if there is such a bias to battle magic solving all one's problems?
I contend that we have built a character not just mechanically (all the items, making our own spells. the staff) but narratively around the rejection of the uncertainty of the standard battle magic list and I would really hate to abandon all that one day because the Everchosen means a big war and a big war must mean big magic.
We don't need to be the only wizard in the army for BM to be the most impactful option. We don't even need to be the only BM capable one. Everchosen are not known for bringing small armies, nor are they known for lacking in Sorcerers. There is going to be plenty of need for BM, possibly more then the entire colleges can provide. Also see below how killy BM can make Mathilde much more effective even if we lean into her skills.
This all feels to me a bit like trying to hammer a square peg into a round hole.
And to me your position feels like desperately trying to avoid hammering a square peg into a square hole because you saw someone smashing their finger with a hammer once. So let's not just fling this sort of accusation around, shall we?
Now, the promised hypothetical:
As the armies are preparing for a major clash, magister patriarchs are tasked with assigning their wizards to the areas where they can provide the most impact. Mathilde, who is both a great fighter and a rider, and is alsocapable of getting her unit through an otherwise impassable terrain is assigned to a large unit of knights. That unit is supposed to counter charge across a bog and hit the enemy in the flank or a rear.
So, the moment comes, they charge through, smash a unit of Chaos Warriors with power of surprise and MMM... only to be confronted with a herd of trolls.
Now. Without pit of shades or other killy BM we don't exactly have much to do. Flask can kill a few, but this is a herd, it won't be enough. MMM will help, but there is only so slow and clumsy a troll can get. We can sword them good, but again, this is a herd, it will take a lot of time to kill them and they will take plenty of knights with them. Worse yet, the charge stalls, the counter attack gets stuck in a grind and we are at a risk of being counter attacked in turn and potentially routed.
With Pit of Shades on the other hand, we cast it and significant chunk of the herd is gone. At that point we can kill the rest with other means and move on.
Or perhaps we are confronted with Hellcannons. Unlike normal cannons those things are nasty melee combatants, maybe we can beat them, maybe not, but in any case prologned combat risks us getting counter attacked and overwhelmed. Pit of Shades will straight up remove them. Or at least destroy most of them so we can clean up fast and keep rampaging or retreat safely.