Petra's plan was quite simple. She wanted to use her signature magic which allowed her to create very lifelike replica of a person from the sand magic and then to use that replica to lure the puppet master from the shadows.
As Sand magic was something that Ren also had and he even had an unique advantage in this matter, Aurora told him to observe Petra closely and learn from her. Petra was a master of the sand magic, so learning from her would only be good for him.
As Petra herself suffered from the restrictions that other sand mages also suffered from, she had a spatial storage specially to carry high quality sand for her use.
So she took some of it out and used her spell. Immediately the sand gathered together and turned into a sand sculpture of her own self. It looked extremely perfect and lifelike, except for the sand like colour.
