Barbara, Fortune Changer

{2}: Scry 3 on target player's deck.
{t}, {4}: Each player looks at the top card of
their library. They may cast it without paying
it's mana cost. For each opponent who does it,
you do the same.
{t}, {4}: Barbara deals damage to target player equal to the number of cards they know in
their library. Activate this only once.