The mini-boss of the Spirit's Grave is a large Ghini. This spectral enemy floats around the room, occasionally charging at Link. The key to defeating it is to attack it with your Wooden Sword when it's vulnerable. The Ghini will often disappear and reappear in different locations, so be ready to move and dodge its charges. It also summons smaller Ghinis to assist it.
Focus your attacks on the main Ghini. The smaller Ghinis can be annoying, but they are less of a threat. After taking enough damage, the large Ghini will eventually be defeated, leaving behind a warp portal and often a chest containing a valuable item, such as the Power Bracelet. Stay mobile, time your sword swings, and you'll overcome this ghostly foe.