Major Magical Focus: Surgical destruction?

I am trying to find a balanced, concise, and relatively clear definition a Major Magical Focus whose rough theme should be surgical (i.e. extremely precise) destruction. The focus should cover a subset of less than a quarter of the Perdo Technique (since a Major Magical Focus should encompass less than half a Form), though I guess spread out over all the Forms.

I'm currently thinking of the following guideline to decide whether a given effect falls within the Focus: would significantly extending, strengthening or broadening the effect ruin it for the majority of the uses it may be put to? If so, the focus applies. For example, a "forget I was ever here" spell falls within the Focus - blasting the target mindless is probably not what you would want when using the spell. Similarly, a spell that carves a statue out of a stone block by destroying some of the stone falls within the focus, as is something that removes the nourishing properties of some food or one that shaves the target. Clenching Grasp of the Crushed Heart, the Wound that Weeps, and spells that poison lakes, deaden storms, or blast demons out of existence do not qualify. Somewhat borderline are spells that cripple the target, e.g. blinding him: I can see a magus crippling a target either to capture/exert non-lethal pressure on him, or to cause as much damage as possible without having the oomph to do more. So, the definition is not very clear-cut yet. Also, it's not 100% clear to me whether it's balanced.

So... could you help me find a sharper, balanced definition?


Is "forget I was ever here" really all that surgical? Compared to, say, "Forget that my cloak was blue?" If I were wearing the GM's hat, I'd want to make sure that the focus does not include every use of Perdo that isn't a major splatterfest.

I don't think I'd allow a Perdo spell that created a statue by destroying bits of stone, since Perdo makes things worse and cannot make things better, and a statue is obviously better than an ordinary stone. So I would also not allow a focus in a subset of Pe to apply.

Maybe Affinity+Puissant+Cautious with Finesse is more apt?

It allows the "surgical" destruction of earth right beneath a target's feet, for example.

A spell that made a statue would require Finesse, so it would help here too. (But the spell would be Rego or Creo, never Perdo!)



You might want a definition like: only Perdo spell, with Part as the target.
It is the closest I can come for a simple definition not bringing too much arguing. Yet it can lead to lengthy argument.