CC dsmash, space and use leg invincibility on dsmash, bair over and hit his head, ftilt him as he rises or from further back just over his sword/at his head if he used any invincibility, space all the way around fsmash, shield grab (no reason it shouldn't work if he's at all invincible when the hitboxes are out). CC shine on the first hit, as long as you actually hit Marth with the shine should work until around 444% (after hit, 440% pre-hit unstale first hit, 442% fully stale), any higher percent should results in a knockdown. If you're not CCing, first hit of nair knocks down at 230% (after percentage, 226% before unstale hit, 228% before fully stale). It's not like you have a tiny window to do it before you start getting knocked down either, off the unstale nair you have 7 frames to do it at 444%. This means you could also dsmash or usmash, but those are harder, and usmash basically has to trade. Grab could also work in the same time frame. This is assuming the nair's first hit hits you on its first active frame, of course.
