Ursula K. LeGuin
Rocannon's World
Rocannon was a Terran scientist on Fomalhaut II one moment, the sole survivor of his crew fleeing the vicious attack of the invaders from Faraday the next.
Marooned among the humanoids of this alien planet - the cave-dwelling Gdemiar, the elvish Fiia and the warrior Liuar - he had to find some way to fight back.
Rallying the primitive natives around him, Rocannon set out to prove that technology was no match for courage and love of freedom.