If you're playing on an Official server or one with the same rates, do not use regular crystal to tame a crystal wyvern. At around 65%, the taming bar started slowly going down faster than it could go up after each feeding. I tested on a level 20. The issue is probably worse at higher levels. Instead you need primal crystal, obtained from KOing other crystal wyverns. Be warned, primal crystal spoils and does not stack.

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