You draw two cards and target opponent discards two cards.
You must be able to target an opponent to cast Mindculling. If that opponent is an illegal target when Mindculling tries to resolve, it’ll be countered and will have no effect. You won’t draw two cards.
If the opponent has one card in hand, that player will discard that card and you will draw two cards. If the opponent has no cards in hand, you will draw two cards.