Way of the Argosi by Sebastien de Castell
Ferius Parfax has a simple plan: kill every last inhabitant of the spell-gifted nation that destroyed her people, starting with the man who murdered her parents. Killing mages is a difficult and dangerous business, of course, but when she meets the inimitable and extraordinary Durrall Brown she discovers that physical strength is not the only way to defeat evil of all kinds.
So Ferius undertakes to study the ways of the Argosi: the loosely-knit tribe of tricksters known for getting the better of even the most powerful of spellcasters. But the Argosi have a price for their teachings, and by the time Ferius learns what it is, it may be too late.
Perfect for fans of The Dark Tower, Firefly, Guardians of the Galaxy, Terry Pratchett, Ben Aaronovitch and Jim Butcher.