Commander Liara Portyr's triggered ability reduces the cost of all spells you cast from exile, even if you are casting them with a different permission. For example, if you are casting an exiled permanent spell with an Adventure in the same turn that this triggered ability resolved, it would cost {X} less to cast.

If you attack a planeswalker but don't also attack that planeswalker's controller, that player won't count toward the value of X.

Commander Liara Portyr doesn't have to be among the attacking creatures.