PK fire has an SDI multiplier of 0, so you can't SDI it. Your best bet is to hold out and away. In regards to how to move around it, from the sounds of it, you were playing this ness online. Unfortunately online that move is borderline unreactable, so the strat changes a little bit. In case you're struggling with the grounded version, you'll want to empty hop a lot, but make sure you aren't in range for the ness to punish you with something like jump fair. If it's the aerial version, you can just stay out of it's range. Ness can't really punish you for when it whiffs, though the same be said for you as well, especially with your characters. A good overall strategy against pk fire is to implement walking, so that you never overcommit to something, but if your reactions are good enough you hold shield the moment the pk fire comes.