Baral's Expertise

If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs, such as emerge costs. You can, however, pay additional costs. If the card has any mandatory additional costs, such as that of Cathartic Reunion, you must pay those to cast the card.

If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

If the Expertise spell you cast has any targets, and those targets become illegal before the spell resolves, the spell doesn't resolve and none of its effects happen. You won't get to cast a free spell.