Chirp

The battle you need to find will be titled "The Darkest Day" and the team will consist of Zacian, Zamazenta, Eternatus, Zacian (Crowned), Zamazenta (Crowned) and Eternatus (Eternamax).
 
Since all battles are randomly generated teams, it is still possible for a normal battle to have a Zamazenta in it. This will not give you the item.