If we're talking about a typical "PK-Fire! PK-Fire! PK-Fire! PK-Fire!" type Ness on For Glory, the best thing to do is use Mewtwo's Confusion to reflect his PK-Fire. Trust me when I say the Ness will most likely not see that coming the first time, and that will give you an opportunity to grab or punish. Watch the patterns in their battle style, and stay on the defensive side. Sometimes it's best to avoid the Ness completely, and remember how they react to your rolls and dodges. From there, you can make further predictions about what they will do when you play defensive. Watch out for Shadow Ball, as a Ness can absorb it and regain health. This one is risky, but when Ness jumps up after being hit off the ledge, it's easy to predict when he is going to use PK-Thunder and recover. If you happen to predict when Ness is going to use PK-Thunder, that's your cue to jump in and: 1) Use Confusion on the thunderbolt and make it go the other way, often leaving Ness confused and unable to use his recovery. 2) Jump right into the thunderbolt and let it hit you. Ness will then go be unable to move (is it called freefalling?) and will end up SDing.
I hate to stereotype all Nesses on FG, though... when battling a more technical and intelligent Ness, be sure to always stay safe. Space yourself properly, and make sure you watch out for his F-Smash. It could potentially KO you at very low percents if you are by the ledge. Smarter Nesses will use their PK-Fire at unpredictable moments, so expect the unexpected. I believe his aerials are quicker than Mewtwo's are, so be sure to bait the opponent to possible get in a strong F-Air. Ness is a really strong opponent, and I still have issues against him. Don't let him grab you at high percentages! Mewtwo is pretty light, so you could be KO'd at around 90% from Ness's back-throw.