How to fix a glitch with the Kina Gratitude Crystal sidequest

The only option that I'm aware of is using the Revitalizing Potion to repair your shield on the field. You can get at the Bazaar from the potions lady. Of course, you'll need an Empty Bottle to be able to purchase it.

Obviously the first thing you want to do is memorize the song. You'll want to know when to expect the fast parts and whatnot to keep it as smooth as possible.

To strum the harp, you're turning the Wiimote left and right more than you're waving your arm left and right. So if it is pointed to the left, it will be playing at the lower strings. Pointed to the right it would be playing the higher strings.

When you're strumming the harp with Kina, you want to be going at a speed where you play every note decently spaced out for each time they wave. If you were to look at their waves and your strums position over time, it should look like this:

When the fast parts come up, it's a lot more lenient. You don't necessarily have to be evenly spaced out like the normal parts, you just want to finish before the next wave comes.

The freestyle part, I think anything goes. I always played through them just random stuff.

If you still need more pointers, I recorded myself playing through it.