Use the powers you got from the guardians, like when Gannon is protected with the fire and tries to hit you, use the goron shield, and when he isn't attacking, use the gorudo spin attack and when is is stunned both times just attack him
magnesia poles get it in lightning before touching thunder blight then hit him when you see electricity on pole. then get ready for long spin attacks and stay back then dodge and flurry rush
There is kind of a glitch if you get him to follow you towards the entrance or exit of the beast you'll notice the ceiling is lower. Because of the lower ceiling he'll be at your level and always stay there. Just hit him a bunch of times.