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
Demon (Demonic) Servants & Enemies
Ramesses II (Ozymandias)
Buff & Debuff - Craft Essences
Elizabeth Bathory (Caster, Halloween)
Nero Claudius (Bride)
Hessian Lobo (Avenger of Shinjuku)
Tamamo no Mae