Normally I'd say Lycanthropy is a little much, but for the sake of the experiment, I'll give it a go.
So let's see what we have. . . .
Initially the plains folk do well, hitting the hillfolk (who always live in the same place) then fleeing before they incur any major casualties. One attempt to actually drive into the caves turns disastrous, however, when the hillfolk shaman uses his or her powers to create powerful flame that scares off the plainsfolk's animal allies and injures some of the plainsfolk themselves. One of the plainsfolk is captured, and nursed back to health by a young hillfolk shamaness who uses her magic to charm him and bind him to the tribe's fire.
Using his knowledge, under the auspices of defending themselves so they can negotiate, the hillfolk tribe is able to build a few fire-hardened spears, and they learn where the plainsfolk tend to hide, so they plan an ambush, preparing torches to set fire to the grasslands and drive the plainsfolk off.
Meanwhile, the plainsfolk have gotten hold of some of the powerful magic that is the fire, and the lead witch tries to learn how to make use of its power, praying to the stars when the animal spirits will not answer her. The junior witch of the tribe is concerned that the elder witch is being tricked by the gods of the hillfolk, and she tries to gather warriors who are loyal to the animal spirits. There is some division in the tribe, since the elder witch thinks it might be wise to steal more fire from the hillfolk, and possibly even make peace.
Thus, when the hillfolk attack at night, bearing torches to frighten off the plainsfolk's animals, they strike a divided tribe, scattering it. The grass does not burn too far, and of course it is easy for the plainsfolk to move to a new spot, but the numbers of the plainsfolk dwindle. They are weakened even more when the younger witch learns that one of their own betrayed them, and though she does not understand the usag of charm spells, she thinks the elder witch may be charmed as well. The witch is killed (though ironically enough not burned at a stake, since fire is now taboo).
To taunt the plainsfolk, a few days later several wolfskins are left at the edge of the forest, draping off branches. The hillfolk have been eating whatever wolves they capture.
In retaliation, the new head witch assumes control of the tribe and grants the animal spirits' greatest blessing to one of their warriors. For three nights, at the full moon, he transforms into a wolfman, a manbeast, invulnerable to simple wooden and stone weapons. The wolfman terrorizes the hillfolk for two nights, killing many wherever he goes, and only seemingly afraid of fire.
On the third night of the full moon, the werewolf arrives with allies, his fellow tribesfolk carrying skins filled with water that they use to douse the guard fires and darken the caves. In near blackness, the manbeast kills many, until one old hillfolk woman stumbles into the cinders of a dim fire, and pulls out a burned shaft of bone, which she drives into the face of the wolfman, cutting him deeply and driving him off. Only a few hillfolk warriors survive, but they have beaten the beast for now.
Now it's time for a quick wrap-up. Everyone post your thoughts on how you think things turned out, or where they might go from here. I see the hillfolk looking at the bone and determining that only weapons that have an affinity to their foe will be useful, and also only those that are blessed by fire. The plainsfolk, frightened of fire and viewing it as evil magic, neglect to use it as a tool to burn down the forests near the hillfolk caves. Because both sides are weakened, the plainsfolk head south early, and are forever wary of fire. On the other hand, the hillfolk's new fascination with weapons created in fire leads to them eventually, maybe a year or more later, discovering the uses of a soft metal, like copper or gold.
