Mephisto and Thor have fought several times. Mephisto has strong abilities that have let him fight with other top tier fighters including Thanos, Silver Surfer, and Galactus. So he’s showed how tough he can be on several occasions. Of course Thor has also fought many strong opponents and even matched skills against Superman. With the Destroyer armor, Odinforce and the other power ups over the years Thor takes this match. He’s simply a better fighter and would take Mephisto down overtime. Mephisto can only take so much punishment before he’s defeated. Thor wins.