Prefered Stages

Small Battlefield

Pokemon Stadium 2

Battlefield

Unfavored Stages

Lylat Cruise

Yoshi's Island

Unova

Always shield PK Fire, then grab him as he goes in for an attack
Try to intercept his PK Thunder with a reflector if you have one
Lots of Ness mains go for either a grab, or do another PK Fire after a PK Fire, so either shield again or go for that spot dodge and get the punish
Always try and intercept his jump offstage then go for gimping his up B
Watch out for pk thunder 2 on stage. If you hit the head of pk thunder it will stop it
Watch out for yoyo on ledge
Ness is very very floaty so force him to jump and take advantage in air and off stage
If your character has a counter, use it against Ness's recovery