Gauche may have some pretty solid mirror magic at his disposal, but it won’t be enough to do any real damage to Asta. Asta’s too fast and his sword completely negates magic so that puts Gauche at a huge disadvantage from the start. He’s just out of luck here. Asta also has his demon mode which he could use if necessary, but I don’t think it’ll come to that. Gauche will go down before Asta’s superior power. Asta wins.