When it comes to theory, assume the worst.
Also we need to start discussing stage choices. I'd say our best against MK is Final Destination & second best would be... Yoshi's Island? Ban Brinstar/RC.post from AiB said:Stay on the ground, Ftilt upwards, use his low aerial movement speed to your advantage by baiting aerial klaws, if you ever manage to grab him chaingrab him and hit him with a FAir, DI and SDI everything upward except when you are hitting his shield (because of his grounded UpB), fortress OoS will beat tornado if he is too low, otherwise just tilt shield up and try to punish his lag, bait him into trying to punish firebreath and fortress OoS, don't try to follow up on any succesful hit (not counting chaingrabs), just position yourself.
The DI part is really important or you will just be gimped extremely easily because of Bowser's bad recovery.
Some players say that it's possible to escape tornado by mashing buttons, I haven't faced MK enough recently to find out if it's true.