OK, so I want to augment this week's nasty Brabacon bandits with someone who can give them an interesting edge in battle with magi (just some little nasty surprise). Who's plausible/cool? I'm at work, so I don't have any books with me, but I can't stop plotting, so help a brother out!
(Of course, it also occurred to me that a Hermetic appreintice who's running with the wrong crowd could be cool.) 
Thanks,
Patrick
Depends. A natural magician giving them a few Fortunam trinkets allows them to reroll combat rolls, but that would be pure mundane attacks: if your magi are well protected against those, no biggie.
IN general, hedgies are extremely bad at beating a semi-decent MR (except vitkir, but those do not run around with bravacons, generally). The easiest thing would be to give them a few magic items looted from a failed hermetic expedition.
Or you can give them superpower from the Magic book if you go with the natural magicians of the book that can grant any supernatural virtue out there. Flying and fire-throwing invisible bandits.
So it depends on what level of fantasy you want in the scene.
Cheers,
Xavi
Thanks, Xavi. I'm imagining something low-power, but I would like something visible enough to get the players scratching their heads... (Although maybe somewhere closer to "Why did that spell not work?!?" than to "Ho-hum, he got a +1 Soak versus my Pilum of Fire, did I notice?") Definitely not flying or invisible.
P
A natural magician can create wards vs specific kinds of damage. If your magi favor fire, the dudes can get wards vs magical flames, for example, granting them +12 soak or so. Adding armor and the like this can counter quite a bit of the damage capacity of your magi. Tailor the defence depending on your player's offensive spells. After all this is a troupe specific encounter and you want to make it slightly tougher for them
As an alternative you can arm the dudes with "grenades":
Mythic herbalism. Level 9. Create a poison that causes a medium wound. +3 to make it a CONTACT poison (12). +1 per extra dose.
Int(2) + MH(5) + puissant MH(2) = 9 + D10.
Assume a roll of 6.
TOTAL 15.
12+3(extra doses) = 4 doses.
The dudes can have 4 grenades for each 3 days that the mythic herbalist spent on the lab before the engagement.
This is a contact poison inside a small clay container that causes a medium wound when it touches the target's skin.
The advantage is that this is HERBAM damage, and it is NOT inherently magical (parma does not protect you against this damage).
So there you go. They can easily have 15 or so "grenades" to rain on your magicians.
There you go with the mages saying "WTF!?!" when ambushed and their initial spells are answered by a rain of clay pots causing medium wounds. Or you can throw chamber pots full of venom and nails just for fun 
Cheers,
Xavi
Xavi, you are an evil, evil man. Thank you.
P
I have been designing natural magicians and folk witches recently. If you restrict the virtues that the natureal magicians can grant to core book virtues only both are extremely solid and cool character concepts. IMO the witches lack the Hex virtue from ROP:I, that IMO would fit them perfectly, but I really like those 2. The grenades idea came to me after trying to see a way to make a witch semi-combat capable. It only works vs living humans and animals, though, so it is not perfect. A barrow dweller would laugh pretty hard at you. If he could laugh, that is. vs living beings affected by humor imbalances it works perfectly.
Cheers,
Xavi
Someone or something that is naturally immune to magic, it just doesn't work on them. Move this into a tradition of anti-magic persons who have whatever agenda you'd like them to have and you have instant bad guy du jour, that still has problems with a rock thrown at him, and throws a wtf at your troupe.
There's the Invisible to Magic Virtue from True Lineages that you could use. That and Craft Magic from Societates would make a pretty potent hedge wizard bandit, he could make trinkets that would protect all of the bandits from magic that targets them directly. Add in Ways of the Forest to really make them shine.