Mostly just edge guarding stuff. For a character that is supposed to be a "glass cannon" he sure does have a hard time landing the final hit.
One thing that works for me is when the opponent tries to recover from below and to the side of the stage. They never expect the Dair to stage spike them. Afterwards, GaW can easily recover. It's one of his better moves offstage if you ask me since it isn't as much of a commitment as, let's say, Toon Link's or Greninja's Dair offstage.
Also, when the opponent is trying to land, do mix ups with Up Air, Up B, Dash attack, and Up Smash. Up Smash in particular is very good (despite the poor range) because it has upperbody invincibility making it a fantastic anti-air attack.