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

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.