6th Singularity Camelot: Land of Emptiness free quest is the best AP & time-saving location to farm Meteor Horseshoe. You'll get about 4~5x Meteor Horseshoe by spending 220AP.
3rd Singularity Okeanos: Caldera Island free quest is the first location you can farm Meteor Horseshoe.
Here is a list of servants that need Meteoric Horseshoe to perform Ascension and Skill Up.
Rashoumon event (Ibaraki Douji event)
Murder at the Kogetsukan
Inheritance of Glory
5 Star Servants Tier List: Best Servants [5/9 updated!]
Best Servants [★4]: 4 Star Tier List
Halloween Strike! Event Missions [JP Halloween 2017]
Best Craft Essence: Craft Essence Tier List
Cu Chulainn (Alter, Berserker)
Jeanne d’Arc (Alter)
Cheer for Master
Demon King of the Sixth Heaven
Talk on the Heated Sands