I've got some bad guys and i need some protection for them.
The idea is that my players mages will be equally matched by enemy wizards, the enemy however do not have the Parma Magica.
However, i do want them to have some defense, preferably in an ablative way (i.e. gets weaker the more you fling at it).
Are there any rules in the game for such a defense?
Failing that, would it be possible to construct a waiting ward with a potent Perdo Vim spell to cancel the effect of an incoming spell cast on oneself?
The aim is to have bad guys immune to the first bunch of spells flung at them (or nearly immune) but become exposed once their magic defenses have been stripped away, giving the hermetic magi an advantage, but not making beating the enemy a cake walk.
A "wizard" without parma would, I think, find his best defense to be proxies. What kind of tradition are you looking at? You could do lots with golems, spirits, mind controlled agents, etc. Not being seen is, despite a fun skit, a good defense that is completely non magical.
Unfortunately I don't have much time right now, so I will keep this short: By canon the access to general MR is just the big trump the OoH has over any other tradition with a Magic or Faerie background, so if your opponents shall be powered by one of these realms the shouldn't get something too close to the original. But here are the solutions I can think of.
All hedge traditions have powers to weaken spells (slightly), but some can actually produce charms to protect from a specific kind of magic or to grant Lesser or even Greater Immunities (cf. the chapters on Learned Magicians & Gruagachan)
Relics grant MR as does the holy Method/Power combo of Invocation/Blessing (only for basically good guys, but this needn't mean that they are not the magi's deadly enemies).
the Infernal equivalent: Incantation/Diablerie
also somehow Infernal: Magic Might by the Infernal Power of Ablation
perhaps (Serf's Parma): protection through a spirit posessing the bad guys.
a Verditius might have develloped Parmulae (Parma artifacts) and started to sell them.
i like the idea that there is some sort of a circuit between the hedgies and a scapegoat. The first few spells cast at the hedge magician affect a different target.
I'd had that as an idea that maybe a agus could bind spirits to himself and use them to absorb spells flung at him. Each spirit would absorb a single spell and then vanish.
Having the spirit binding as a ritual would prevent too many of these spell sponges from being practical.