The Guide by Awaki Kobone (粟木こぼね).

This story follows a young prince named Boots (the youngest of seven brothers). His six older brothers set out to find brides, but on their journey back, they encounter a cruel, magical giant who turns the brothers and their new brides into stone. Boots sets off to rescue his family. Along the way, he encounters a starving wolf and shares his food with it. In gratitude, the wolf becomes his magical companion and trusty steed. The wolf carries Boots through mountains and wilderness until, as shown here, they reach a hidden mountain lake. In the center of the lake is an island with a church. This location holds the secret to defeating the giant. The giant cannot be killed by conventional weapons because he keeps his heart hidden outside his body. The giant reveals the hyper-specific riddle of his mortality to a captive princess: "Far, far away in a lake lies an island; on that island stands a church; in that church is a well; in that well swims a duck; in that duck there is an egg, and in that egg there lies my heart..." With the help of the wolf and other grateful animals (like a raven and a salmon), Boots crosses the lake, retrieves the egg from the duck, and squeezes it to destroy the giant, breaking the curse and saving his brothers. George W. Hood, for the 1922 book The Norwegian Fairy Book.