If MK is approaching from the ground some options you have are F-Tilt or pivot grab. They counter MK's dash attack, tilts, SH f-air and running grab. If the MK is tornadoing low enough and your timing is perfect, you can pivot grab, u-smash, or d-smash the nado. But you are better off just sheilding and punishing. When above MK in a juggling situation, hovering down airs are a good way to deter air chasing and poorly place chuttle loops. Inhale is also great landing tool as long as you mix it up with other options such as inhale or air dodging away.
Your main focus would be to defend and punish the MK while taking note of its habits. You have to try your best to slow down the pace of the match and not let the MK toss you around the stage. A good stradegy is to f-tilt repeatedly to box out MK's tils then punish the option they choose to get around f-tilt. A lot of players like to power sheild the f-tilt then punish. You can anticipate this and grab them. You need everything you can get vs MK so perfect your tech chases. Thank and work on defending yourself from nado.