I loved the thing, and wanted to do some templates for fun, but never had the time for.
Rough ideas: If you're a mundane, to kill magi, you need to either be very close, or out of sight. You also need to be able to take them out quickly: Either they'll kill you, or they'll magic up something, even if it's just a Bind Wounds spell.
To get close, you need to be either very good at stealth, or very good at mundane disguises. For more Advanced/paranoid magi, the Persona ability may be a boon. You also need to be very good in your chosen weapon, to inflict debilitating wounds as fast as possible.
To kill them, steel is enough... for junior magi. I figure Magi Slayers would consider weapons made from rarer materials, like animal or human bones. Depending on how your saga treat wards (Does a ward vs steel also protects vs stone?), stone weapons may also be used, including stone knives. There's always the possibility of weapons made of "precious stones", even if these were created via a CrTe ritual spell.
It is, of course, always possible that they are equipped with 1-shot Charged items with high-penetration spells, although, IMO, this detracts from the psilos purpose and advantage, which is to free up magi time and attack in non-orthodox ways that may be countered by resistance mastery. These also require high lab totals, for both the base spell and the penetration (Consider PeCo 30, +1 touch, +25 for Penetration 50 is already lvl 60, and I'd consider these to have at least Pen 100 to be relatively "sure shot", which means lvl 85)
Consider PeCo effects, of course, but also PeMe, and most Creo spells. Creo Vim is great for elder mages.
Poison their food, their water.
Killing their mundanes attendants may be a good option, for the more junior magi who are unable to teleport back to safety: Stranded in the wild, they may wander out of the game, or, hopefully, starve to death.
Set up a forest fire. The fire probably won't kill them, but the fumes may, and you may deprive them of food.
Set mundane traps. IMO the best ones are pits. Charged items may be great there: plant in their stuff an item that creates a huge pit at the next sunrise/Sunset (we know items can react to such environmental changes in the flow of magic). If the pit is big enough, chances are they're toast.
Consider working with other mundanes, using the stuff from A&A. They may create poisons, acid, fire, which aren't magical, and thus non-resisted. Other exotic materials may be useful too.
There are lots of things people don't ward against. I once had a magus come across a specialist in the exact same form he specialised in. So I ran in and punched him (using the scuffling rules) until he was unconscious, and then drowned him in a barrel of beer because I was pretty sure he hadn't warded against the nearest drink.
A good idea for a Tremere magus-slayer - the covenant servant. Take someone with the skills you would see used day-to-day as a servant, train them in guile and stealth, and make them learn Finesse by using magical labour-saving devices (magic brooms, clothes-presses, etc. from covenants). They look like regular servants who can help out in a covenant...but give them a magical device designed to avoid Parma by requiring a Finesse roll, and they can ambush a magus.