Not sure if this applies to every type of wyvren including the other subtypes and general wild ones, and perhaps I just stumbled on some lucky glitch, but to easily kill specifically a tamed, unridden tropical crystal wyvren is to just crouch and shoot arrows at it, as it cannot reach the ground enough to bite while you shoot it from below. This is perfect for especially low-level survivors to take out enemy tribal wyvrens that have no riders on them to activate their special abilities, and endlessly loop above you but failing to damage you if crouched. Just accidentally discovered this on The Island and was on a pretty flat piece of land so not sure how elevation plays into it. This is my first post here and any positive attention is appreciated. Thanks!