I play a lot with a decent Ness so I'll offer some advice on that MU. If you have any other questions, please ask. I'm sure I missed something.
Dealing with PK Fire:
DON'T SIT IN SHIELD. Ness has stupidly good shield pressure from PK Fire, very good grab options, and his aerials. You REALLY don't want to shield much. Stay moving, dash dancing is very important because it makes it easier to avoid PK Fire and forces him to come closer when he uses it. Trying to reflect it doesn't do you much good either. It travels a set total distance, so you need to catch it at the very beginning if you expect to hit him with it, in which case you're already hitting him with Nayru's itself
Recovery:
Generally, DON'T USE DIN'S. PK Flash covers a huge range and is a scarily good edgeguard option against floaties with poor air mobility *cough*Zelda*cough* so you usually just want to forego trying to place a Din's to help you recover.
Neutral:
Getting in close with Nayru's is good, because it really screws with Ness, and you typically can get a good follow-up, whether it be a kick, a grab, or just a dash attack. You need to alternate between weaving in and out in the air and dash dancing on the ground. Mixing up your horizontal spacing is crucial against Ness, because although he can quickly cover lots of vertical options, he has to commit a little more fully to actually going in. Ness will prefer to zone with f-air, PK Fire, and a few other moves. You want to bait and then punish.
Actually using Din's Fire:
"Oh, it's Ness, he can absorb Din's Fire, blah blah, it ruins my stage control, bleh!" Yeah...bull crap. It's not hard to effectively use Din's against them, you just have to know how to do it. You have to realize that Din's is just a trap against Ness and Lucas. You WANT them to absorb it. After absorbing a move with PK Magnet, there is a little bit of lag before they can do anything. Your goal is to force them to run into that Din's with PKM and be close enough to come in with a grab, kick, etc. Din's can also be used to edgeguard, which is explained just below
Edgeguarding:
First off, get used to how far it travels, especially that little bit at the end that most people don't expect. You can easily have the tables turned on you by edgeguarding improperly. Ideally you can go out and edgeguard him offstage. You can simply jump on top of him and take the hit of PKT, forcing him to fall to his death (MWAHAHAHAHAHA) or you can try to kick him before PKT2 starts. This is VERY risky if you aren't ready to do it, because PKT2 is powerful and has 7 frames of invincibility at the start. Assuming you can't get him offstage...Din's is still good. Ness runs right through Din's with PKT2, but he probably doesn't know that
There is the TINIEST hurt box that might bump into it, but it probably won't ever happen. Anyway, he doesn't know that, so use Din's to influence the direction that he comes in so that you can throw out a well-spaced kick. It is very possible to kick him out of PKT2, but it takes some practice with the spacing.
Other/the tl;dr:
Don't get d-aired. Kick a lot. Grab a lot. Use Din's appropriately. If he d-throws you, he can get damage follow-ups if you DI forward and kill follow-ups if you DI backward. If you don't DI, he gets his choice. Make it your choice. Nayru's is good close range, but don't use it to reflect PK Fire. Don't shield PK Fire. Spot-dodging PK Fire is probably your best option against PK Fire. Notice a pattern here? PK Fire can ruin your day.