Nearly every Shadow in Persona 3 Reload has an Affinity weakness. Targeting this weakness will Down the enemy, opening them up to an All-Out Attack. As you climb higher in Tartarus, you'll run into gatekeeper bosses with several Affinity resistances and no weaknesses. One example of these bosses is the Arcanist Decapitator, a gatekeeper boss on the first half of the Yabbashah Block. You'll unlock the first half of the Yabbashah Block on July 18th in Persona 3 Reload.