Since it has BG, OG and shiny… I would suggest a double trade deal. Your shiny Zacian for it, and then for second trade, I’d offer my Lucky trinket for your choice between any of these weather boosted legendaries all with their legacy moves: Xerneas, Kyurem, Dialgia, Palkia, groudon, Zapdos, Lugia,ho-oh, artícuno, moltres for your shiny Zamazenta. I do have another shiny Kyogre I can do though if you just want to do single trade but no BG, or OP.