If an opponent’s library contains fewer than X land cards, all cards from that library are revealed and put into their graveyard.

If another spell or ability instructs you to cast Mind Grind “without paying its mana cost,” you won’t be able to. You must pick 0 as the value of X in the mana cost of a spell being cast “without paying its mana cost,” but the X in Mind Grind’s mana cost can’t be 0.