Phase 1: Attacking the Shadows
Shadow Hag is a ghostly boss that appears as multiple shadows on the walls of the room. Only one of these shadows is the real Shadow Hag, and it will periodically emerge from the wall to attack Link. The key to this phase is to identify the real Shadow Hag. The real one will have a slightly more defined appearance or will be the one that directly attacks you.
When the real Shadow Hag emerges, quickly use your Seed Shooter equipped with Ember Seeds to shoot it. The Ember Seeds will cause it to solidify and become vulnerable to your sword. Attack it with your sword while it's solid. The other shadows are decoys and cannot be harmed.
Phase 2: Direct Combat
After taking enough damage, Shadow Hag will stop appearing as shadows and will instead fly around the room as a single, solid entity. It will charge at Link and occasionally shoot projectiles. Continue to attack it directly with your sword. Use your Roc's Feather to dodge its charges and projectiles.
Keep up the pressure, and Shadow Hag will eventually be defeated. You will receive a Heart Container and the Echoing Howl (Essence of Time).