There are many enemies in the Zelda universe, but sometimes they are hard to figure out. Due to the controls of Skyward Sword HD, they are even more unique. One such enemy is the giant spiders.
To defeat giant spiders depends on where they are located. If they are on a single web thread dangling from the ceiling. You need to swipe with your sword once, and it should move and spin simultaneously. What you need is to make it, so its back is facing you. On its back is a purple circle, and this is its weak point. You need to use a stab/thrusting attack, which for motion controls is thrusting forward with the Joy-Con, and with buttons, you need to press the right thumbstick in. You need to do this a few times, and it'll eventually die.
If the giant spider is on the entire web itself, there is no way to spin in. You have to get behind it and stab it in the purple circle a few times, and it'll die.
A few times, you have to face the giant spider when it's on the floor. This is the most difficult way to engage it. You have to attack it until it goes into a defensive stance, with most legs protecting its face. When it does this, you have to slice upwards, from below to above. If done correctly, the spider will fall on its back, where you have to do a fatal blow finisher to kill it.
We have more the Legend of Zelda guides here at PGG.