Please help! Game keeps freezing at Dragon Ruins

Hi! My game keeps freezing at the Dragon Ruins of the Taen quest part of the game. It started when I intentionally fainted all my pokemon so that I could return to a pokemon center to buy enough pokeballs to catch Zygarde. I had my last pokemon faint, scurried to a pokemon center, bought some heavyballs at Helios city then went back to the dragon ruins, repeated the double battle with Taen and noticed that it would freeze every time the cutscene where Zygarde attacks Zekrom happend. I loaded the other save backups and every one of those back ups ended up with the same results. Could someone help me find a solution for this please? I really dont want to start a new game from scratch:-(.