Main use of Feint L is to counter wind element mobs' increased evasion property.
Feint L is a very important Logos action to have for the fate The Baldesion Arsenal: Expedition Support. After two waves of mobs fate boss Tristitia will spawn. Immediately apply Feint L to the boss and never let it drop.
Feint L is also very helpful for the Absolute Virtue fight inside the Baldesion Arsenal. It can also be used in Pyramid Shape of Ozma fight, although having a Spirit of the Remembered negates the evasion effect gained by Ozma.