Now go back where the Plateau Point started. There are two empty grey pedestrals that need to be filled. One is nearby, the other one close to where the blue cannon challenge was. Put them on the pedestrals and push the box onto the yellow square to get the first chest.
After the battles, you end up next to an empty green pedestral. Go back through the battle grounds. The statue for it is next to the battle entrance. Put it on the pedestral to get access to the two chests.