Suppose I'd like to do the exact opposite of the Creo Corpus magic used to increase characteristics?
This sounds like the realm of Perdo Corpus, since it makes the person worse than the essential nature. It may also be unnatural, but this doesn't feel like Muto. Muto can do other things, like shrink people.
Also, I'm aware of the non-permanency. The Creo magic needs to be ritual, but for Perdo it may be better to just require a non-momentary duration in order to make the magic efficient. Sure, even a brief reduction of strength may be fatal to a strongman holding a boulder above his head, but...
What guideline to use? The ones for Creo magic ridiculously high, but need to be that for balance. Base 5 "Hamper a person, i.e. make them lame, blur eyesight" sounds like the absolute minimum. And perhaps this kind of magic needs to be less general like entire characteristics but more specific like the examples above.
So "lame" would yield a penalty for moving around, but wouldn't affect the Dex for craft or other hand-coordination. And "blurred vision" affects anything vision based, also archery, but not any other of the senses. As for the size of the penalty, that is left up the the Troupe it seems. I'd go for a -3.
For use in combat, to hamper an enemy, you could cast a spell to sap the strength of their arms, reducing damage. To weaken their backs/core strength to increase Encumbrance etc. At -3 per pop.
Although a penalty to Str across the board would be nicer, but indubitably at a higher Base. But how high? Base 15 is equivalent to "destroy minor sense, cripple a limb".
Perhaps Base 5 for -1, and a further magnitude per extra -1, up to a maximum of Base 25 for -5?