I wouldn't send out a Pokemon with such a horrible disadvantage in a 1-on-1 anyway. Except when I sent out Rawk Hawk against all those Dragons and such in my All-Comers' Challenge... Even then...
...Well, I guess the short answer is that it CAN be done. But it'd require more cheese than Wisconsin's entire industrial history has produced in order to pull off with any form of consistency. You'd have to have plans for EVERYTHING the opponent would be throwing at you.
And I'll admit, off the top of my head, I can't think of a way for a Leavanny to take down a Magmortar. I'm not Batman. Leavanny is part-Grass, so access to a move like Sleep Powder or Grasswhistle would probably be one of my earlier ideas.