One of the protagonists of HBO Ars Magica has a Wizard's War declared against him. Because this is in Hibernia, the War is essentially open-ended in its duration. The aggressor NPC is a mature Flambeau magus (Igneous Drake) with the predictable abilities if you took the sample Flambeau magus in ArM5 and advanced him into his 70s (casting total of around +100 with CrIg, most other Ignem spells in the book, and a few utility spells like Bind Wound and some defenses). His familiar is a flying drake the size of a lion with Might 20 and fire-breathing. The target is Cernunnos, a newly-Gauntleted but well trained Ex Misc beast-master. Igenous has figured out that Cernunnos can tell when Igneous is coming (the PC has a raven companion with Premonitions, but Igneous hasn't quite figured that specific out yet), and when the PC figures this out, he just hides in the covenant's Aegis. Igneous has one idea: one of Igneous's sodales is Oswald of Bonisagus, master librarian at Elk's Run. Oswald has arranged to visit the PC's covenant on a book-exchange mission. Oswald will bring with him a shield grog and a servant, and possibly a scribe in case the visit lasts a season and a book needs to be copied.
How can Igneous exploit this mission of Oswald's to the target PC's covenant and gain access to his target, WITHOUT asking Oswald's permission beforehand? (He's perfectly happy to apologize for it afterwards). He could probably bribe or threaten either the servant or the scribe into cooperating with a plan, if he had one.
well, on the obvious spectrum is to convince one of the mundanes to carry in an item that Abysmal Flames the PC's sanctum. Any item Drake could craft should be easily able to overcome the Aegis protection, and even if the PC wasn't in the sanctum, he just lost most of his magical power. The same applies to Drake himself, he should be powerful enough to just walk up and flame the offender personally although that will cheese off the other covenant inhabitants.
On a more subtle note, just have the mundane collect arcane connections during his trip; this allows the PCs to have a chance to detect and foil the plot without directly confronting (and dying at the hands of) Drake.
With CrIg of 100 or so, he can WW the whole covenant if he likes.
Or use the mission to extract an AC to the PC. A signature on a document would do, say, a receipt of some kind. A book trading or scribing mission offers all kinds of possibilities.
With an AC, a decent penetration and a CrIg spell at range AC, it's a done deal.
Of course, you have a big problem if the plan is too good..... /2
(It would be even better if the NPC transformed himself into something that was taken into the Aegis, such as a scroll or a flea, but his magic probably doesn't run that way. Still, CrIg offers other possibilities, such as simply obliterating the cathach, which is completely legal in Hibernia. Without a cathach, the covenant isn't legal, so the magi of the covenant have no special rights to the structures or library or real estate since there is no legal covenant, so collateral CrIg damage is not likely to deprive any other magus, unless they themselves or their apprentices or familiars are injured, which is not likely to be a problem with Mastered CrIg spells and some Finesse.)
Would the Aegis Cernunnos is 'hiding' in hinder Igneous Drake's spellcasting in any relevant way? From the OP, I assume not.
Does Igneous Drake know the location of the covenant? And does he know that Cernunnos will 'hide' in the covenant when he approaches? Then he might just waltz in, as he apparently has little means to track down a magus in Ireland. Inside, his main problem would be, to find and dispatch Cernunnos without drawing other inhabitants into the fight.
And here Oswald and his entourage might come in handy for him. Oswald's presence will likely stop the other magi of the covenant to attack Igneous Drake without declaration of Wizard's War, unless the Flambeau caused excessive collateral damage. And Igneous Drake could keep this - by his standards - low, if he could induce a member of Oswald's entourage to help him spot Cernunnos within the covenant.
Like Ovarwa says, if the antagonist has CrIg casting totals > 100, then he can do whatever he wants. Even without an AC, he can easily kill anyone at the covenant.
BoAF at a casting total of 100 has 65 levels of penetration built in, excluding the penetration ability or mastery bonuses. An Aegis 25 takes that down to 40, outside of the Aegis. To be protected from such a character, within the Aegis would require a MR of 40. I looked up the character, he has Parma Magica (Ignem) of 2, and an Ignem score of 1, so he has an MR of 16. Cernunnos should be dead whenever Igneous wants him dead, at least mechanically. Were Igneous to step inside the Aegis to cast his spells, the situation becomes even worse for Cernunnos, as the Aegis only take 13 off the penetration, not 25.
I think it's reasonable, from a character/world perspective for Cernunnos to have some idea of the capabilities of this magus based on the character's knowledge. Does he know how old Igneous is or is it easy to find out and then calculate his casting score magnitude within a range (such as 15-20 magnitudes in casting power behind his offensive Ignem spells). Once he realizes the vast differences in power, he should understand that hiding behind the Aegis really isn't doing anything significant, and that Igneous Drake wants something from Cernunnos that doesn't involve his death.
So, Igenous could just walk into the covenant to visit Oswald, setting the covenant abuzz. And then after casually visiting Oswald he knocks upon Cernunnos' sanctum door. Just knocks and waits for an answer. He might also have a spell that makes a room uncomfortably hot...
To better answer the question, we must understand the motives behind Igneous, and how they align with the character you've sketched mechanically.
Referring to a recent conversation about balancing encounters, this is exactly the kind of thing I mean. There was a lot of talk about always letting the chips fall where they may, or ashes, in this case. But then there is no saga. Because unless Mr Fire Drake is an utter idiot, this WW can be won casually, and then all the work you spent in Season 1 is wasted on a dead PC, which sucks if you have to keep paying the actor . But that's only if he wants to win. Maybe he wants something from Cernunnos, like JL says. Or he wants to test his mettle for some longer term plan. Or maybe he declares WW on everyone, and then kind of forgets about it: How long will it take Cernunnos to realize that there will never be an attack? Or maybe Igneous takes advantage by sending a bloodcurdling letter... and doing nothing else?
Of course, the PC might have done something that reasonably, logically and aptly ought to result in his becoming a smear of ashes. Game of Thrones in Hibernia, ftw: We do kill off your favorite characters. (Is that the kind of HBO saga you wanted? /2)
Flambeau to Tytalus: You get further with a Pilum of Fire and a cunning plan than with a cunning plan alone.
It looks like Cernunnos just has to wise up about Igneous Drake. By the OP, Igneous Drake doesn't appear to be able to track Cernunnos, or find him outside of his sanctum. So Cernunnos can hit the many roads and byways of Ireland every time his raven companion has a hunch, and thus be safe.