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!]
Best Servants [★4]: 4 Star Tier List
Best Craft Essence: Craft Essence Tier List
FGO Reroll Tier List [5/11 updated!]
1-3 Star Servants Tier List
Cu Chulainn (Alter, Berserker)
Jeanne d’Arc (Alter)
Altria Pendragon (Lancer)
Tamamo no Mae (Lancer)
Black Holy Grail (The Black Grail)
Piece of Year 2030 (A Fragment of 2030)
Faithful Dogs Can't Wait Anymore