DI up, take the worse followup, laugh in his face when he's offstage. Catch his double jump, get hit by the PK Thunder, and he's usually dead. A lot of Ness players are cripplingly addicted to jumping and spend a lot of time in the air, so focus on those options once they realize PK Fire isn't safe on shield up-close; if you know what you're doing it's super easy to parry due to the sound cue.
You've got em, champ.