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.

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