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
4th Singularity: London
5th Singularity: E Pluribus Unum
How to Build a Buster Team
How to farm EXP cards & level fast
Piece of Year 2030 (A Fragment of 2030)