@Boney : I had some thoughts regarding your musings on the potential limitations and problems with the Staff of Mistery and spamming Battle Magics.
Thus far, in every case where we've used Battle Magics, it's only ever been singular and focused. In other words, we would cast one spell at the right place and time, and maintaining that spell for as long as it is most useful takes up a significant length of the battle itself. We didn't cast Miasma repeatedly and often against an entire army, we cast it once on a key enemy formation until that formation was dead. In a more hectic and chaotic battle, I could maybe see us casting it briefly and repeatedly against very small groups of enemies that we would teleport into and then cut down with our sword before moving on to the next one, but that frankly constitutes Battle Magic being cast down at very small scale in a more niche circumstance.
If we were to invent more fog/mist-based Battle Magic, I imagine the limitations would be several-fold before the Staff itself became the bottleneck (or lack of one). Creating a fog-bank of poison-gas would obviously take time to generate the fog bank and spread it to where it needs to go, and then take more time for the poison to debilitate and kill the targets. As such, the Staff wouldn't let Mathilde spam the spell at all, since the spell itself would take focus and time to be effective in the first place.
Any truly
fast fog/mist based Battle Magic spell would have to be very limited in what it could do by its very nature--sure, maybe you could create a huge fog bank to obscure your army from view in a pinch, but that's
all it could do, since if it did damage you'd have to be very careful about where it's placed and the possibility of friendly troops wandering into it. If it allows friendly troops to see through it but not enemies, it'd take time to spread to an effective size and need to be maintained for a length of time for it to make a difference on the battlefield. If it confused and disoriented the enemies within the fog bank, it would need to be spread and maintained for a length of time for it to have a meaningful impact.
In short, the limitation of the Staff of Mistery of being limited to fog/mist spells (and the reasonable levels of power of Battle Magic as opposed to Cataclysm-level spells) should serve, along with QM discretion, to be a sufficiently limiting factor before the staff reducing the risks of casting such Battle Magic becomes something abusive. Fog/mist spells seem to be innately effective
over time and with concentration rather than one-off magical artillery blasts, so spamming them would be self-defeating.