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.
Saber Wars: The Caliburn Awakens Rerun JP 2018
5th Singularity: E Pluribus Unum
From Mar 15 ~
5 Star Servants Tier List: Best Servants [5★]
Best Servants [★4]: 4 Star Tier List
FGO Reroll Tier List
1-3 Star Servants Tier List
Jeanne d’Arc (Alter)
Cu Chulainn (Alter, Berserker)
Altria Pendragon (Lancer Alter)
Black Holy Grail (The Black Grail)
School of Chaldea