Lucius Cypher said
Centuries? Can Kossiths even live that long? I can imagine a blood mage getting ageless imortality, nit nothing else about the other classes would suggest extended longevity. Plus Arcane Warrior is an Elven magic. I'm surprise the kossith, who are typically magic-phobic even outside the Qun, would know about such arts. Though I'm suspecting artistic licence for how or when kossith set up any sort of village in Fereldan(Given thats where Flemmeth lived most if her life).
She was in a form of stasis. Basically a painful stay outside of the flow of time. Also, the colony being there is actual canon. Disappeared in First Blight, some female Kossith were turned into broodmothers, thus making ogres.
At -410 Ancient a group of kossith landed in the southern Korcari Wilds and established a colony. <--wiki for the Kossith, so was already there
As for learning Arcane Warrior, she could hav eeither seen it done and developed it or have learned it from elves in the area she could associate with. I did not feel a need to explain it really, but I can come up with a part of why. In addition, the Kossith are not necessarily one group or mindset. She was in a group that was not magic phobic and that likely would not have converted to Qun.
I did make a clerical error as Ancient is in negatives so I did the wrong direction of 20 years.