Felicia's Plate + AoE Special — does it work?

The description of the special will always take precedence. It is also supported by the fact that AOE specials trigger before and outside of combat.

Just tested this and it works. The AoE damage will target the lower of the opponents Def or Res

AoE specials already took the lower defense to begin with, so it would be the same with or without Felicia's Plate.