Our saga's in the dying years of the Schism War, and PCs will soon be hunting down remnants of House Diedne. But the setting means the only Diedne left alive are the ones who've survived this far. They'll be tough to beat!
if I'm GMing those folks, what will the Diedne have done to survive a few years of war? What will they do to survive an encounter with a typical Flambeau hoplite?
So far, we've got
A decent Parma Magica
Invisibilty spells to avoid being targetted by sight
Use of personal-range "Ward against Hermetic Magic" to back up the Parma Magica
How about 'Forget you ever found me' spells -- especially helpful for dealing with mundanes and intermediaries, who can then be prevented from giving clues to your whereabouts or activities. Or a similar 'Hidden Glade' type spell which makes your domicile unfindable.
Hard to enter regios. Bonus points if arcane connections don't work across the boundaries.
Shapechanging.
Focuses in dispelling combined with Diedne non-fatiguing sponts. Probably with a Formulaic Group target version of Unravelling the Fabric of Ignem that will work on CrIg 35, mastered for Fast Casting, for Flambeau specifically.
I think one of their best way to fight off without killing would be to give warping to the assailants as the Diedne were strong on Creo & Vim magics
But keep in mind that the Diedne were heavily reliant on spontaneous spells as they were really good at it. In the end of the war, the survivors were probably good at escaping more than fighting. You are probably dealing with non-renounced Magi that are harbouring the Diedne and I would expect by then that quite a few would fight you legally with Wars and such to protect their amicus or simply persecuted Diedne. I see Merinita & Bijornaer to be sympathetic to their situation along with a bunch is ExMisc and a few paficists of the other houses.
For Hermetic magi, offense is vastly stronger than defense: with a little preparation one can easily incapacitate an opponent with a single magical blow. So, you want your enemy to strike first ... someone else, thereby alerting you he is around (and ideally wasting resources and revealing additional intelligence about himself). Then, you can strike him from the shadows.
Of course a crafty enemy will himself use decoys to strike at your decoys, tricking you intto striking his decoys so that he can strike you. The game then becomes one of being able to layer more levels of deception than the other, so as to have the last strike.
I've got a non-obvious, potentially totally novel way of surviving. Someone had Harmless Magic. A few magi arranged to have this one mage disintegrate everything (the people, their items, their Familiars, their vis, etc.), including themself, in a Room for Year. Everything reappears a year later. They repeat the ritual a few times. Alternatively, everything in a Circle for Ring, and there is a Watching Ward holding some Intellego effect and something to break the sturdy, unknown-to-everyone ring much later.
Another obvious tack, though probably not what you are looking for, is timeshifting.
Whether by entering a regio in which every day is a year outside, or entering Faerie and accumulating a sufficient Fable score, it is not that hard to leap forward a century or two, when people have probably forgotten all about you and you can pretend yuo are just another harmless Ex Miscellanea.
Have someone with Nyctophylax (from the Tremere section from True Lineages) there to fight someone trying the old "surprise them at sunset/sunrise when their Parma is down" tactic. There's a reason why the Tremere love having apprentices who show this tendency.
Persona. Maybe it's time for the Tytalus who infiltrated House Diedne to go back home. Or is it a Diedne infiltrating House Tytalus? Does it matter when you have more than one personality?
Frankly, another good reason to survive is being an apprentice who didn't meet the order of Hermes prior to the schism war, and your parent never coming back. If no one knows you exist, you're hard to track.
This cropped up in our current game. If they meet Huw again (or other Diedne), what would those folk be doing to stay alive? (Assume that, for whatever reason, the PCs and the Diedne are in the same place at the same time.)
Only one thing I want to point out: Mastery - Magic Resistance. With only 4 seasons (or a couple of Andventure/Exposure XP here and there) one can have 4 spells mastered for Magic Resistance.
1 CrIg spell with +damage type base.
1 CrVi spell with Warp inducing base
1 The Call to Slumber
1 The Wound That Weeps
Because of the "Similar Spells" rule you have effectively doubled all your MR scores against CrIg fires spells; Warp inducing spells, spells that make you sleep and PeCo spells which cause direct wounds/bodily harm.
We can safely assume that a mid-level mage has Score 5 in all the Forms (at minimum) and a Parma of 3. That means the mage has MR of 40 for the effects above. If you raise the Parma to 4 or the Form to 10, you have MR of 50. That is... more then enough to deflect these direct attacks.
A more subtle way of security is more realistic tbh - but my experience is that it is hard to grasp for the player what is going on behind the scenes when the enemy is really subtle, smart and spy-like, and that can be frustrating for them. It can easily be seen as some kind of fudging from the GM/ASG's side.
I once too on a challenge of designing an unGifted redcap who would challenge full magi to Wizard wars to gain respect. One of his favorite tactics was taking refuge in a church, with mechanical booby traps at his usual locations. The wizards are all looking for spell traces and magical traps when a crossbow bolt hits them in the back or the ceiling collapses on them.
While this is a very good tactic, I think its not quite as good as you lay out here. Spells are similar if they have (ARM5 101)
• Same effect, at a different Range, Duration, or Target. All three may differ.
• Closely related effect, at the same Range, Duration, and Target.
Two spells have the same effect if the rules description of the spell is the same, apart from the Range, Duration, or Target. Closely related effects include such things as doing damage with Creo Ignem, or turning a human being into a land animal.
So Pilum of Fire (+15 damage, R Voice) is similar to Ball of Abysmal Flame (+30 damage, R Voice) and to Pilum of Sight Fire (+15 damage, R Sight) but it is NOT similar to Ball of Sight Flame (+30 damage R Sight) and it is NOT similar to Circle of Encompassing Flame (different damage and duration)
So mastery of Pilum of Fire does not actually double your MR against all CrIg fire spells.
True but it does cover the most popular ones for which all the mastery books are written for which are the ones you really have to worry for. I mean a Magi with 10 mastery in Pilum of Fire is scary... even if you have mastered the spell in resistance. You better use a ward vs fire in case the MR fails.
It's anachronistic because while sun Tzu has been written it has not appeared in the west, but
Know the earth
know the sky
know your enemy
know yourself
And you cannot lose. Of course the far greater and less quotes advice is "the greatest victory is to turn an enemy into an ally" So master those ReMe spells.
Not specifically your character: there are other dangerous magi out there. Building defences specifically around your character would be an unfair GM move.
I think we do not stress how powerful this is. The Diedne fast cast defence against pretty much any effect thanks to their un-naturally powerful spontaneous skills. You basically need to outnumber them to have any chance of getting a spell through if you are facing them directly.
Best you find a mundane way to attack them or take them by surprise or secure arcane connections and finish the job remotely. Face to face, their fast cast defence vs magic is just too strong & if on the offence they have an effect that beats your Parma, you're un real trouble.