Daily Quest: Monster Hunting Saber (Monday) Advanced is the best AP-saving location farm Phoenix Feather. You'll get about 1-2x Phoenix Feather by spending 300AP.
Daily Quest: Monster Hunting Archer (Tuesday) Expert is the best time-saving location farm Phoenix Feather. You'll get about 1-2x Phoenix Feather by spending 400AP.
Here is a list of servants that need Phoenix Feather 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!]
FGO Reroll Tier List [5/11 updated!]
Best Servants [★4]: 4 Star Tier List
4th Singularity: London
Fate Grand Order Farming Items
Tamamo no Mae
Archer Inferno (Tomoe Gozen)
Kato Danzo (Katou Danzou)
Jeanne d’Arc (Alter)
Trick or Treatment
We'll Conquer the Ocean of Stars
Castle of Snow