I'm going to ignore the extreme difficulty of finding the right Magi and Covenant. Things like a questing Flambeau hunting hedge wizards might have traveled from several countries away, most Magi are basically unknown outside of the Order because they avoid interacting with mundanes (Gift), and it might not be any living people outside the Order (including Covenfolk) who even know the Magi exist. Magi do live long enough that several generations of mundanes will pass during their lifetime.
Covenants generally do not advertise what they are and many are difficult to reach. That difficulty might be out of the way location (mountain top, underground cavern), any of a large range of magical effects to conceal them, or even an almost impossible to enter location (Regio, on a cloud, under the ocean).
Generally against mundanes this is the first and most effective defense. It might take a lifetime of work to find out which individual Magi it was, what Covenant they are a member of, where that Covenant is at, and how to enter it.
While a nice initial thought, trying to infiltrate a Covenant is not normally an effective strategy. Outside of hiring specialist or when conducting some large scale construction project, most other than Spring have a fairly stable population of covenfolk that might have been serving them for several generations at the least. Even if you do manage to join you will be an outsider and thus stand out if you do anything or go anywhere.
Just managing to become a covenfolk also does not mean access to or interaction with the Magi. Because of the Gift most of the Magi will act through the varies leadership positions which means that while most of the covenfolk might know them by sight few will actually interact with them.
Magi will also often have a personal servant who handles things like cleaning and their food. Attempting to poison them will often require poisoning a large gathering. This does vary by Covenant but is generally not an effective strategy.
Attempting to attack them in their sanctum is also not an effective idea. Without some supernatural aid all it takes to stop is a single Ward verse Mundanes.
That leaves us with attacking them outside the Covenant somehow as the most effective strategy.
If they are a Magi who travels fairly frequently or does something like go to a church outside the Covenant then you could setup an ambush. However they might not travel for a few decades or they might travel through magical means. So the chances for this are highly dependent on the individual Magi and a chance might not come up during your (effective) lifetime if they go into a heavy period of study or enchanting.
Which leaves us with inciting against them. Depending on the target and what they are doing for the next few decades this might actually be the only method that has a chance of being effective. A multi pronged approach that gets the locals, the church, the fay, and any nobles in the area unhappy with them is the best bet. It might take years and it will harm all the other members of the Covenant.
Magi by their very nature are a "Hard" target. Individually very powerful, strong security that is often layered, restricted personal access, and generally very obscure to those outside the Order.
Actually having a chance at succeeding is highly dependent on the target doing something that makes them exposed or going with some scorched earth method that hits the whole Covenant.