>>54970872I don't remember the exact moment but he fought it alone before Zacian and Zamazenta I'm pretty sure.
Look it up, the timeline is
Calyrex defeat eternatus alone>Wounded Calyrex reach crown thundra >Calyrex meet his horse>Calyrex move the forest to protect them against Eternatus>Zacian and Zamazenta fight eternatus because Calyrex is too mindbroken to fight it again