You must equip the Rain Miracle, or the Centura won't appear. As soon as the stage starts, start building up your Fever. You will want to enter Fever mode before getting too close to Centura. When Centura is invisible, none of your attacks will work. After you perform the Rain Miracle, Centura will become visible and vulnerable. Besides losing Fever right before the fight, this boss is not that hard. See "Centura" for more details. At this point your army should be strong enough to kill Centura before the Rain Miracle ends. If you can't nail the Miracle, go to a hunting stage and practice until you can confidently perform the Miracle at will. Upon defeat, Centura drops Lv.1 Egg of Centura to be used in the Patagate.