You have to buffer,but if they are low enough that you cannot hit them you should use Dtilt to raise them up or make them jump to retreat.You should use Fair if it's damage and nair as a high percent kill move.Although at low percents down throw into nair is a good option.
Here's the thing...about killing
Option 1)Down Smash - it's always predicted and you're opponent can just stay in the air but this is where Nair comes in handy.
Option 2)Grounded Shuttle Loop - it kills characters such as
and
early.
but there is another use for Nair (let's say you are on
and you are facing that Zelda).
If Zelda is on a platform above you you can Uair juggle and Nair finisher.
As for gimping and you are ready to knock them away so you can recover you should use a fresh nair for the following reasons : Fair doesn't send them too far and Dair could accidentally send then the opposite direction (assuming you just went offstage).