New fan love my first 2 ff games but stuck on x

I first got into ff through crisis core reunion and vii remake. I loved it. I wanted to play x-2 and xiii next but was convinced by other fans to do x first. I am having an incredibly hard time. I made sure to constantly switch characters to level up everyone, but I don’t have any special weapons, my Aeons feel weak and i cannot get pasted the 3 phase Seymour boss fight for the first time when you face him and anima. Does anyone have any advice? My current save everyone has overdrive accepted yuna at the start of the fight. His little auto-healing guard are shutting me down. Any help would be greatly appreciated.