Bronze Key Captain:
There are three types of Earth Wyrm attacks. First, it attacks by slapping down its body, you can dodge to the side to avoid this type of attack. Secondly, it attacks by soaring and chopping at those around it. When that happens, you'll want to be as far away from it as possible. Lastly, it attacks by calling down waves of power from above. When this happens, it doesn't matter if your close or far away. You'll die instantly if you don't evade it well.
Ah, one more thing! The Earth Wyrm's tail is its weakest point, so whenever it makes an odd action, quickly attack its tail.