Plax Protetor

This is not the same as hexproof. If, for example, you target one of your opponent's creatures, your opponents won't be able to target their own creature with spells or abilities.

If the target creature is an illegal target by the time Shielding Plax tries to resolve, the Aura spell doesn't resolve. It won't enter the battlefield, so its enters-the-battlefield ability won't trigger.