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.
5 Star Servants Tier List
1-3 Star Servants Tier List
Christmas event: Almost Weekly Santa Alter [2017 US]
Best Servants [★4]
FGO Reroll Tier List
Christmas event: Merry Christmas from the Underworld [JP 2017]
Caster of Midrash (Queen of Sheba)
Black Holy Grail (The Black Grail)
Piece of Year 2030 (A Fragment of 2030)
Holy Night Sign