EDIT : And I am interested in your theory. Please post it whenever.
Okay, so in essence, people have identified a very salient point. Battle Magic is dangerous.
First and foremost, it is incredibly dangerous to our enemies. Under its sheer force, brave companies of soldiers can be reduced to so much ash, flights of arrows redirected or rotted in mid air, the keenest steel armour turned to rust, and more hideous things besides. It is a primal and devastating power, and those that wield it are truly terrifying beings when in their element. Armies may crumble under the might of battle magic. But magic in Warhammer is also inherently dangerous to its users, and Battle Magic most of all.
As a brief refresher, magic in Warhammer draws from the Winds of Magic, a primal force of raw Chaos ultimately emanating from rifts to the Realms of Chaos at the poles. The whole practice of magic is about
forcing this raw chaos to obey a set of ordered formulae, usually a spell or enchantment, by utilising incredibly specific language sheer will to
force it into a shape with no room or error. Magic inherently does not want to obey your stupid rules, because chaos dislikes order, and will
fight you the whole way. The splitting of magic into eight Colours with somewhat predisposed characters is the one thing which makes this even remotely possible for human wizards.
The other is the language that Imperial-trained wizards use for their spellcasting,
Linguae Praestantia, which was taught to the first Imperial wizards by Teclis of Ulthuan. It is essentially a dumbed-down version of the Asur's own magical language,
Anoqeyån. This language
itself a devolved version of that heard spoken by the High Elves when they met the Old Ones. The key factor in each of these languages is their immense complexity and specificity. Almost any thing, state, or process can be codified specifically, and this means one can construct watertight magical formulae without ambiguities, which are as dangerous in spell-casting as ambiguities in nuclear safety systems.
One could liken this to a computer program, which will not run correctly or may develop bugs if one has left ambiguities in the code. Except even CSS does not cause demonic snakes to try to eat your face when you mistakenly left a recursion error in one line of code.
To wield Battle Magic, then, is to to try to channel and then
constrain immense quantities of raw magic into a specific form, under battlefield conditions. The practitioner must do this in within a short time frame, and potentially whilst people are trying to kill or interfere with them, increasing the chances of making a mistake. Furthermore, the magic itself
wants them to make a mistake, and will search out any weak link or loopholes in their casting. Thus unless they are very experienced, or the spell itself is exceedingly simple and straightforward, this can lead to problems.
But there is a partial solution to this problem which is well-established in the the source material.* These are call Bound Spells.
Bound Spells are spells contained within a specially enchanted item which contains all of the formulae necessary to contain and craft the magic for a very specific spell. They may even take care of the channelling, as well, and perhaps even contain a power source. They are immensely potent and valuable artefacts, because at a low end, we are talking about things like a ring which can cast fireball, reliably and relatively safely, whenever a specific word is spoken,
even if the user is not necessarily a wizard. To return to our computer analogy, a bound spell in an artefact would be like a USB stick with an executable program all ready loaded and ready to go. Plug and play.
However, it seems to me that there is not a binary distinction here. It should be possible, in principle, to create magical foci and "crib sheets" which vastly reduce the mental strain on the practitioner*, by containing much of the necessary formuale for a
single Battle Magic spell. We could also build in safeguards and extra reinforcement and reservoirs for challenging large amounts of magic, in one artefact or perhaps several artefacts. This would not cast the spell for us, but greatly aid a practitioner in casting a given spell safely and easily, with the drawback that we would need these items.
For a wizard like Mathilde, who does
not want to spend her entire life on battlefields as living magical artillery, but is a gifted enchanter with time and resources, this seems like a good option*.
Just as one example of where we might go with this, one of the players remarked to me earlier that we have gained a wolf pup from Ranald as our familiar.
There are apparently a lot of options for using familiars as a magical foci. Imagine if we invested several years of work on this, building our connection with our familiar and deepening the enchantments on it. Imagine a fully grown wolf familiar, wreathed in magical shadow like smoke. Imagine our night-black wolf companion transforming at a single word from Mathilde, into a gigantic Steed of Shadows, a great Black Dog out of nightmare.
*(Other than having a divine patron do a lot of the "computation" for you, which Priests and some other kinds of spellcasters do.)
*(If one is familiar with how magical foci work in the Dresden Files, one may be intuitively familiar what I mean here.)
*(Actually, it seems to me like something along these lines would be a good option for most Wizards in the middle of their careers who are not "front line" Battle Wizards but would like to be able to cast a single Battle Magic spell when "called up" for active battlefield duty. This interpretation could go some way to reconciling tabletop and roleplaying canon, but I digress.)