Just a little thought experiment. Let's say you have an entirely mundane person by Ars Magica's standards - no Gift, no inborn Supernatural Virtues (but any that can be obtained by such a person after the fact are fair game), who wants to make a life hunting supernatural creatures - all the sorts of Magic, Faerie, and Infernal entities who bring humans suffering.
Let's assume they're well off enough, free enough, and knowledgeable enough to pursue most any avenue to skills, resources, and powers available to mundane folks by Ars rules, with two exceptions. First, we'll be excluding methods like the Odin sacrifice or Garden of Eden to gain the Gift, because "just become a magus," while probably the most powerful strategy possible, defeats the fun of considering what non-magi are capable of. (Initiation into Virtues is fine in general, however.) Secondly, while they are better off than most, perhaps due to a sponsor, they do not have the ostentatious freedom of wealth and temporal power of higher nobles - no throwing an army at every foe or starting out dropping the worth of a manor in silver pounds on a jacked up Hermetic enchanted item (though they are free to use vis obtained from success in their ventures however they see fit, including purchasing services from Hermetic magi).
The most straightforward answer purely in terms of mechanical utility is True Faith, if you're the sort of personality who can swing it, but given how unreliable, action-limiting, and not really compatible with having your own ambitions that tends to be (it's not really the sort of buff you choose to acquire for other purposes so much as a byproduct of pursuing faith for its own sake far beyond the degree achieved by even most devoted contemplatives), what sorts of other preparations would be worth pursuing as a pragmatic and motivated monster-hunter? What skills, tools, defenses?
Naturally, if you're going the "acquire vis to buy magical support" route, you'll probably want to start small, on low-Might creatures who aren't too insurmountably threatening, and use the vis you gather from that to gain methods of defense against the dangerous shutdown abilities of higher-Might threats. After all, your biggest concern, as someone most likely lacking Magic Resistance, isn't physical attacks themselves, which you could dodge or survive - it's supernatural powers that instantly create a Game Over situation if you're ever in position for the creature to use them. Mentem effects are perhaps the most common source of instant loss, so I'd put a high premium on getting immunity to those via item or Greater Immunity Initiation, but that's just a start. Maybe try to become a half-Taltos for dealing with incorporeal foes? Very relevant if demons or ghosts are among your intended prey, since it isn't subject to Magic Resistance. For that matter, whether any of your offensive equipment should ever be on the enchanting table is its own question, given the risk of having something so expensive to produce be rendered useless when it most counts, though shelling for a Verditius to make an Item of Quality sword is probably worthwhile if you ever find your combat output inadequate, given how comparatively vis-cheap it is.
That's all just cherry picking as a start tho. Let me know what you think are the urgent considerations and things to plan for and strategies and all that!