The full-body transformation art Wu Di used was called Saintly Ape. He gained this transformation art in a fortunate encounter during a mission. Actually, he was not so fortunate in the mission. His mission was to track and kill a mid-superior spirit beast terrorizing a secluded small town. He almost died during the mission.
The spirit beast was very sneaky. It often took its chance to attack the town when the town soldiers were out hunting for it. When the soldier returned, it had run away. Hence, the mission was handed to the Royal Academy. The soldiers remained guarding the town while the royal academy student went to track the spirit beast.
Wu Di saw this mission as a chance to measure himself against a mid-superior beast. From the info, the spirit beast was a loner. He believed he could defeat a mid-superior beast if it were one-on-one.