There are a lot of options here. The wording might refer to a cult - some kind of zealots. Perhaps from multiple separate cults, with the military aspect being the unifying factor. Some might be some kind of "knights in a black armour" furthering the good of all with any means necessary. Some might be in it just to kill some infidels.
However, the way the text presents them, it seems to indicate that they might commonly work alone or supported by "regular" forces - which might be that they've either been picked from a larger group, or then just are that great that the crown sees them as an asset. In here, great does not necessarily mean that they all are individually superior fighters, it might be that one of the zealot warrior cults focuses on that, going out to hunt the enemies of the King, assassinate them, fight wushuesque odds and win. Others might in turn have completely different kind of training - tactics, strategy etc. that makes them useful to lead fleets. Grouping both under the same label, if the inner workings of the group are kept sufficiently secret, make them look even more dangerous. So you'd hear that a Mashtor single-handedly defeated a hundred-strong Blood Raider fleet - and another actually lead the related operation to crush the nearby Blooder bases. After establishing such of a reputation, a few such individuals could swing battles simply by the effect they have on morale.
And of course, the above is completely my own invention with very little basis in PF except the words "sect", "bounty hunters", "not uncommon to lead a fleet".
Edit: "mashtor" possibly means "teacher". Have to check. No idea if this is a play on "master" by some "I speak mumble-jumble so I sound foreign" or an actual word, but it popped up in a Google search.