7th Singularity Babylonia: Marshland free quest is the best AP-saving & time-saving location to farm Serpent Jewel. You'll get about 2~3x Serpent Jewel by spending 210AP.
Rashoumon event (Ibaraki Douji event)
Murder at the Kogetsukan
Inheritance of Glory
5 Star Servants Tier List: Best Servants [5/9 updated!]
Saber
Best Servants [★4]: 4 Star Tier List
Merlin
Chloe Von Einzbern
Divine Banquet
Fate Grand Order Farming Items
Ereshkigal
Archer Inferno (Tomoe Gozen)
Sometime in Summer