So ive been killing gardians that fly, that are in Hyrule with both anchient arrows and my master sword and I'm short one core to get my anchient Helm how do I know if they will drop a core
Ancient Cores can be gotten from any and all guardians that are not broken, you've got a better chance from the skywatchers than the stalkers, some shrines will have them in them as rewards.
Try in the maze north of Akkala lab for the sheer number of guardians or the Major Tests of Strength