Another nasty trick is to make a few charged items I would say one would be a rug which when stepped on activated a huge PeTe effect which would basically close off the mouth of the cave and bring most of the cave mouth down on top of him.
Maybe something which converts lots natural water into a solid form, then when the spell ends (such as a preplanned unravelling the form of Aquam) the items change back into water.
Maybe you could lure him into a strong dominion aura. His magic will be at a major disadvantage, and if you have some handy-dandy grogs, you can have them dispatch him. I have seen a Tremere beaten by a younger holy magus in certamen, simply because the duel was fought in a church-yard.
If you can't face the magus head on, don't. Use terrain and situational advantage.
I wonder whether a month in a monastry might be time well spent. The aura will help and the prohibition of interfering with the mundanes also. A fresh magus is probably not too weird (even with the Gift) not to get the opportunity as long as he donates generously.
I have seen even fake labs created that are basically an armed high penetration nuke. They were only used to show the sanctum symbol and hold the AC to the real (far away) sanctum. So yes, some people have defences. They also tend to be the ones with a proficiency in combat spells. Mental lunatics. I have a pair of them IMS
If you are in such a dire strat and CANNOT allow the abuse to continue, the best way to get rid of the bastard is to engineer him into blatantly breaking the code in front of a trigger happy hoplite. Evil-smelling people are great for that, specially if tyhe evil-smelling people are important and related to the hoplite by blood. If you can't achieve that, put a few mundane trapes in your sanctumand hide somewhere else. I always liked the item holding around a ton of highly Corpus-corrosive acid floating on the ceiling, that deactivates when a Parma Magica enters the lab and suspends the magic holding the acid in place. The same result can be achieved with lava, but wards against stone and flames are more common. Make sure the havoc does not affect the rest of the covenant. Somehow. Destroying the nearby village in the process tends to be preferable.
You cal also hold a few hundreds of asps in the ceiling instead if your enemy is weak with Animal. It is less destructive overall. Getting the asps or the acid should be done by someone else, not you, so people is less prepared for what you are doing.
Plan C is to be in Durenmar studying in the library for the month of the war. be IN the library during that time. I guess the magical defences around would put a serious problem for someone trying to kill you around with magic. Have a pair of highly trained shield grogs with you while studying there. It is a variant of "run" but it can also be turned into a trap to your advantage.
Try to get the older magus wizard marched by provoking him into attacking you in some non-lethal way (e.g. reading your mind to obtain information you won't speak). Then teleport quickly to the quaesitors and have them run an InVi spell on you to detect the attack and the wizard's sigil. The hoplites won't have the same trouble with your enemy that you would.
THat is not a good defense: during the wizard's war, he can do whatever he wants: he has immunity against any infraction towards you.
It includes: killing you, scrying on you, abusing you, mind controlling you...
In short: you are doomed.
And, if he is really old and powerful, there is many chances he is not alone. He can send a faerie / magical friend. Why botter in person? he could even ask your covenant mates to deliver you if they don't want their covenant to explode during the next moon because, at the moment he declares war to the whole covenant, the covenant is an adequate target for his boundary fire spell. Say bye bye to libraries, vis sources and anything you ever owned.
Do you think that magi, who are not friends (only to their familiars) will hesitate?
No, you're right. Obviously it does no good to provoke the magus during the wizard's war. This only works as a pre-emptive strategy. The original post said:
Slightly bigger magi are the enemy and they have not declared war. They may never declare war, content to push you around safe in their superior fire power. Magi who are arrogant and not afraid to throw their weight around are perfect targets for a little manipulation...
While technically true to an extent, it is not true in practice. A wizard is only immune to infractions against the one he's fighting. Actions taken can attract Wizard's Wars and other actions from other wizards. If you tear up the weaker wizard's covenant while rooting him out, odds are the other wizards of that covenant might have an issue. The Order itself though does not have an issue. Such a wizard might find himself a target of several Wizard Wars at once or a long succession of them, as well as certamens to establish dominance.
Take a look at the Wizard's War sidebar in Societates for the House Flambeau section.
WW is also lined out a bit more in HoH:True Lineages, the Guernicus chapter.
But meanwhile, read AM p 14, "Wizard's War". While not specific, it's pretty clear - "step outside the bounds of The Code". But nowhere does it imply that a mage in a WW can do anything to anyone - it's implicit that The Code is shed only with regard to the target of the WW.
So, if mage A declares WW on mage X, then magi B and C (in A's covenant) and magi Y and Z (in X's covenant) are neither involved nor "Code-Free Targets" - WW is no excuse for involving them or their "magical power" (labs, grogs, collectively owned libraries*, etc), intentionally or accidentally.
(* Outside the sanctum of the mage involved. Anything in the sanctum is considered forfeit - and what were you thinking anyway, it's a freakin' war zone!)
It's not unlike 2 countries declaring war - neighboring countries are not involved nor open territory, not without declaring war on them as well, or them declaring sides and joining in the conflict. (But it may be wise for them to strengthen their borders and stay low anyway until the conflict is resolved.)
To the original question: since you have some time to prepare, you could make an enchanted item and hide it in your opponent-to-be's covenant. If it's there when they cast their Aegis, then you can use it without penalty therein. Could surprise the complacent bastard....