When has this ever not been the case? If you can't kill someone with a level 20 spell then you're not thinking about it very hard.
I don't see how that would be especially useful (even ignoring the fact that BtS is range touch). Do you want to take down the parmas of multiple magi at once? The success roll is spell level + simple die you can't spam it hoping to get a multiple 1 die roll to achieve success. You can't take down a parma a little bit at a time with this guideline it is a boolean success /failure thing.
The rest of this post feels much more combative and adversarial than I actually feel. Apologies in advance. We don't actually disagree strongly. It's just that getting into the nitty gritty details can seem like lecturing and I don't have the eloquence to make it more friendly
Let's look at what our apprentices need to do to take down a parma 6 vim 10 archmage's parma. Their spell (again assuming the range voice alternative) needs to be level 20 to be certain of success and they need a penetration of 40.
They have a stamina of +3
Do they want to do wizard's communion? I'd say that this would be inconvenient unless they are using opening the intanngible tunnel or some similar spell. If they do use an arcane connection spell they need to penetrate with that as well meaning that their rego would need to be as good as their perdo and they'd need to expend the same amount of vis again and they give the archmagus something to dispel to protect himself (WOMS is target room they don't even need to cast invisible eye revealed).
Lets say that in addition to focusing on stamina they put a great deal into penetration and have penetration 6 (perhaps skilled mentor and affinity penetration of a lot of xp and puissant penetration).
They could spend four seasons one getting vis one opening a device for enchantment, a third attuneinng the item as a talisman and a fourth instilling an effect in order to open up an relevant attunement. What attunement would be best is there even a good shape material bonus that applies to removing parma. I think that there is a +3 to vim somewhere let's use that (attunements do not stack)
In addition to the penetration virtue lets give them another virtue relevant to the spell perhaps beneficial circumstances or method caster or cyclic magic for an additional +3
Lets give them two levels of mastery for 15 xp -three seasons of practice or perhaps a season of practice and a quality 10 text (that took a season worth of work to acquire so three seasons either way).
(2 points casting total and two more penetration for penetration mastery)
We'll include a few pawns of vis lets say three for a +9 to their casting score
Aura is going to be the same for both the apprentice and the archmagus so its a wash
The apprentice needs a casting total + penetration of 60 they've got 28 so far add in 3 for confidence and 2 for a die roll, they still need to have 27 levels in the arts to have a good chance of pulling down the Archmagus' parma. That's a few more years worth of work. And now magus a can take down the Archmagus' parma and magus b can use the opening provided to assassinate the magus.
And it only took two to four years of study of arts and penetration, a year to create a silly one dimensional talisman, a rook of vis or more. three seasons spent studying spell mastery, a season or two to learn or invent the spell, and a few minor virtues. Now perhaps it might make more sense to use more vis (and get more botch dice) to reduce the investment. Even so this doesn't seem bad to me. If you have a young magus who wants to invest a few years into getting a single chance to assassinate philipus niger that could be a great story. Archmagi are specifically the targets where it makes more sense to go another route than tearing down the parma. The costs of parma dispelling rise faster than the cost to increase parma.
With an arcane connection and a few sympathetic connections you could do it much more easily, but if you have an arcane connection and some sympathetic connections you don't actually need to take down the parma to penetrate so this becomes a pointless exercise (or I guess even more pointless).