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
How to Complete Master Missions [US May 7 - May 13]
Best Craft Essence: Craft Essence Tier List
1-3 Star Servants Tier List
Arc 2: Cosmos in the Lostbelt
Saint Quartz Farming Guide
Edmond Dantes Event Door 7 Arrow 2 Guide
Black Holy Grail (The Black Grail)