Shouldn't it be "The Wizard's Trebuchet" or "The Wizard's Ballista"..? Arbalest sounds a bit dinky to me... 
Earlier this year we discussed the Trebuchet. You can likely search the site using that as a key word. The Trebuchet is likely an item where the rules of Ars break down and can be excessively abused. However I'm not that worried because I don't think seige weapons are a casual occurance in most games.
The issue has been regarded in the past.
I support the interpretation that stipulates that both Aegis & Parma are not purely hermetic Vim spells & hence cannot be brough down by a Purely Hermetic PeVi Base effect as Unraveling the Fabric of [Form] based spell.
You would need to use the base used for wind of mundane silence to bring down a Parma/Aegis or use a PeVi (With mercurian knowledge i.e breakthru) with the specific form Vim.
The guernicus might have that spell somewhere but it would be closely guarded as it could bring down the Order in the wrong hands...
W
The Wind of Mundane Silence is a canonical pure-Hermetic spell which can bring down Parma, right there in the core. It's was around in 4th, too. Note that it has to penetrate the Parma (and Vim resistance), then the spell itself has to be double the Parma; basically the casting total has to be 3x the Parma, not counting Vim score (or penetration bonus). An Aegis should be at least as tough, so you'd need a Wind of level 40, plus 21 penetration to take down a minimal Aegis. Level 40 Aegis would take a level 80 Wind plus 41 penetration. That ain't easy, and I'd think that's not even a particularly strong Aegis by Autumn standards.
And since Wind is defined as Target Room you might rule a special, larger version is needed to target the boundary. Or Ritual, as people suggested. (Then again, you might rule the target is the Individual Aegis, making it easier, perhaps plus a size boost for "large" individual.)
A 8th magnitude aegis is very expensive, and only appliable to the powerful autumn covenants. Anything greater than that is so expensive that it is pretty much pointless.
IMO spells like this pose little problem for the setting, but rather opens up for story ideas - Anyone inventing an anti-aegis spell would obviously be looking to attack ones fellow Magi, and will be looked upon with extreme suspision - and be a target of Wizard's wars.
A fallen aegis should also be pretty obvious to the Magi inside it, and it can allways be be recast. (And so, once your aegis falls, you should prepare for an attack, and recast it as soon as possible).
It has been suggested that since the aegis is not purely hermetic in origin it cannot be dispelled with normal PeVi, but would require a breakthrough. If this is the case then by extension the magic invented by Bonisagus can only affect the magic invented by Bonisagus, and not non-hermetic magic.
If it isn't possible to destroy non-hermetic magic with PeVi without a breakthrough then it shouldn't be possible to change it with Mu or control it with Re or investigate it with In without breakthroughs either.
I think the situation where the players confront a hedge wizard and discover that since his magic isn't hermetic they can't dispel it or investigate it or deflect his magic missiles with Re is ludicrous.
PeVi magic should in my opinion be able to dispel any kind of magic anywhere without requiring a breakthrough. With a few possible and very rare exceptions I haven't considered. Albeit it may require very high level ritual spells.
Initialy I like The Great Sun Jester's spell alot although I may concede that the base from wind of mundane silence might be nescesarry instead of the base from unravelling the form. Not sure though.
In any case in my opinion a breakthrough certainly isn't nescessary.
Incedantally I would also think that an Infernal version of an Aegis (assuming such a thing exists) cast by diabolists around their hideout should also be able to be dispelled with PeVi magic without requiring a breakthrough.
Hail Eris!
Flarg
I like this idea a lot. Or perhaps a Demonic Arbelest. Because then it essentially becomes a plot device. The ability of the players to use and misuse the spell no longer becomes an issue, unless they enjoy being diabolists themselves.
This made me think of a truly evil and devilish plot.
A demon might train some diabolists into creating a spell that has the PeVi effect of being able to take down an Aegis.
The diabolists are happy as they have been given a weapon to use against magi like they requested of their demon benefactor.
The information is recorded in notes/journals/books/diabolists minds/large rock monlith or tablets ( if you fear a Flambeau destorying the knowledge to easily ). The diabolists usage of this spell will cause grief for the characters but the characters are expected to in the end win the day.
The characters are happy because they defeated the diabolists ( pride is wonderful ). They will recover from the diabolists the knowledge to recreate the diabolist spell.
What Bonisagus could refuse to make a great big new break through of magical knowledge for the Order of Hermes ( a little bit of pride and envy mixed in here ).
The creation of the spell will eventually tempt the players into usage or demonstration of this power. The rest of the Order will now start to react with possible fear, calls to sieze the knowledge from the players, possible certamen duels and wizard's wars get to break out.
The whole Order could start to fall apart as the spell gets spread around or people fear that covenants may have the spell.
Chaos reigns.
Happy Demon 
Salvete Sodales
Agnar wrote:
In order to construct a spell to dispell another, there are two versions in the PeVi guidelines (p. 160)
"Wind of the mundane silence" is constructed with the second version, and if the target is adjusted, there should not be a problem if you want to cast it against an Aegis.
But as this would require a rather high-level spell (and some penetration to boot) the first one looks more promising. And as obviously mere theoretical knowledge of a kind of magic is sufficient (e.g. that shamanic spirit control stuff) we should assume that any hermetic magus should be able to construct such a spell against an Aegis - a spell he can actually cast, even though he is a bit weak on some cosmic rules behind the ritual.
By the way, a 8th magnitude Aegis doesn't seem all that rare. Considering the mass of supernatural opposition every sane magus should be interested in a protection as strong as he can get his fingers on. And the stability of the order is guaranteed by the fact that hermetic magic is stronger in the defensive sector than in the offensive one. So fractions willing to keep that stability should be more than glad to spread high level Aegis spells throughout the order. By comparison to other rituals of comparable magnitude they will be easy (and cheap) to get. If there is a resident magus able to cast the spell, the convenant will use it unless it is nearly ruined (i.e. out of vis).
Vale,
Alexios ex MIscelanea (aka Lars Gerlach)[/b]
As for the vis, 6 magi with lousy CreoVim lab totals can distill 12 pawns of Vim vis in a season. I don't know what PC covenants tend to be like but compared to the nearest equivalent, monasteries, one season a year in service to the covenant is a great deal. (Or compared to normal jobs, taking two "seasons" out of a year, distributed, leaving two for study.)
Disregard this post.
Only when a covenant has nothing else to do may all such services be used for vis-extraction. Our covenant can usually only spare 1 or 2 magi for this. And we need the vim-vis for several other uses as well...