Just give Udyr the Ezreal treatment

Take away spell effects triggering on his abilities, then runic echoes becomes unviable on him and he still has 3 other jungle enchants that work on him.

By directly nerfing runic echoes, you hurt every other AP jungler with it even though most of them are in a good spot. By directly nerfing Udyr's base damage/shield you hurt his other play styles.

Except he'll still be op with these changes and then you will gut him with increased mana costs, cooldown increases in between stances, and bear stance/passive nerfs and make the only semi viable play style involve runic echoes. Then in a few months runic echoes will be removed entirely for some other jungle item and Udyr will remain in his grave.