mach tornado is good against rob until he figures out which of his attacks out prioritize it, which would be his charged gyro and laser.
his forward smash is tough because of its range and strength, but there are a few ways to punish it, if spaced just right, you can get a down a past it, if he misses with it you can dash in on the final frames for a grab, or of course you can always come from above with a trusty dair
his nair is strong and lasts a deceptively long time. as said above, the best way to counter it is to either hit him during its start up frames or shield and wait for it to end. one thing i must emphasize with this attack is that if he hits your shield with the initial explosion, YOU ARE NOT IN THE CLEAR! this attack will hit the area where it starts again when it ends, so watch out for the full attack
some general tips against rob:
his up b nearly makes edge guarding impossible, most likely you'll have to get him to 100+ damage and get him with a shuttle loop. dont give up on edge guarding though, its a great way to intimidate and rack damage while your opponent frets
both robs forward and up smashes have good range, damage, and speed, but both only attack in one direction, either ahead or above, so make him choose, try aproaching diagonaly from above, if he fsmashes, get him with a dair, if he up smashes, land and hit him with a ftilt or a dsmash or whatever you like
other than that i try to stay just outside his fsmash range and dtilt/ftilt him into the air where MK's air game dominates