I think there's a few different situations where AOE damage via battle magic would apply:
1) Mathilde alone against large numbers. Fighting is not a good idea in the first place, but if necessary she's got the flask. This is exactly what it was for. Still, running is generally the best choice. Could be improved, but there's no real hole here.
2) Mathilde and a bunch of heroes against large numbers. Either swording or MMM would do well. Very often, the people she adventures with are mages anyway. And she can still help everyone run away. Could be improved, but there's also no real hole here.
3) Mathilde and a small force against another force. Here she still has the option of MMM, though in some cases actually killing the enemy could seriously improve things. IMHO, this is the spot where something killy does the best.
4) 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.
So, my take: The first two options can be improved by battle magic. But I'd say integrating her lesser spells with her abilities has greater impact and reliability. For example, fighting in pall of darkness (which is actually a good combo with our frequent adventure buddy Johann), or learning to use the teleport or Cloak Activity. Or even shadow sword. Point is, these are situation below the scale of battle magic, and so are better solved with other abilites.
For the last option, I don't think there's much use at all. Mathilde is already plenty busy in these situations. Like, doing counter-spelling is something she's actually damn good at, and she has MMM as something to do in between. Or, depending on the situation, leading a flanking maneuver with RoW.
So finally, Mathilde at the head of a small army fighting another small army. No other magical caster for her, and maybe none on the other side. Now, MMM is still a really good option here, so there's no critical lack here. But there are times where just killing a whole bunch of the enemy is more useful than making a wholer bunch of enemies worse. I don't think this is valuable enough to be high priority for a number of turns, but it might be nice to fill out eventually.
Now, you might say: Burn, I just want to do cool magic, even if it's not optimal. Which is very fair. But it leads to another question: Which magic is the most optimal? Also, which is the coolest? Let's consider.
First, from the spellbook: Steed of Shadows, The Enfeebling Foe, The Withering, The Penumbral Pendulum, Pit of Shades, Okkam's Mindrazor
Steed of Shadows is a utility spell, and frankly RoW is better. It's got basiscally no use to Mathilde. It could be cool if the drake it summoned actually did anything. Maybe for rescuing a friend in a tight spot? 2/10
The Enfeebling Foe and The Withering are both debuffs, which given that MMM exists and gets boosted by our staff, is just unnecessary. Also not all that cool. 1/10
The Penumbral Pendulum is a damage spell that applies in a line. So there's some use. It's got a cutting theme, so that's neat. If you want to kill normal dudes and not nasty bastards, a pretty good option. But also not super on theme for Mathilde, though working with liminal spaces might help that. At least an option to consider, and kind of sweet, but frankly doesn't measure up to the other contenders. Useful if you want to get into the SP ring, since it will kill but leave a corpse. 5/10
Pit of Shades is one of the nastiest things you can throw at someone in the TT. It's got a cool liminal space thing going on too. Downside is that it's also one of the hardest thing to throw at someone. Neat, but frankly I don't want to fuck with this. 7/10
Okkam's Mindrazor: One of the best buffs on the tabletop, and also one of the coolest. Weaponize courage. Mathilde often fights with dwarfs who have a lot of courage to weaponize. That's sweet. On the other hand, it's really far from Mathilde's themes, and super hard. I really don't want to fuck with this. 6/10
But Mathilde also can create her own spells! So what might be contenders here?
Choking Fog or Acid Fog. Both of these are options, but they fill essentially the same role. It's a damage spell, but as mist it gets the Mistery boost, which makes it a lot safer. Also cooler. Pit of Shades would likely deal much more damage, but being able to throw it way more freely is a huge advantage 8/10
Riders in Red summon aka Nazghul aka Pocket Posse aka Dämmerlichtreiter. For all those who are in it for the apparition binding. I'd say this is mostly used in the scenarios 1 or 2 above, but if Mathilde grabs a bunch then having a surprise cavalry charge is a powerful move in all but the largest battles. Cool aesthetic, and a lot of history here. I'd say we should aim for this eventually, but maybe not as a first step. 8/10
Painful fog, but with apparition. Whispering Darkness and Black Essence are both contenders for this. This is for those who want a fog spell, but also want apparitions. It's efficient. Also, people getting chocked by black stuff coming out of their face that only they can see is hella metal. And of course, as a mist it gets boosted, so it's relatively safer. 9/10
In conclusion: I personally don't see the need for AOE damage BM. But if that's your thing, I'd suggest you ally with the apparition faction and push for Fearful Fog With Friends. They're pretty numerous, so they'll probably win the vote soonish, and it's a question of which apparition (and therefore spell) takes the lead.
