Lucario's a fun one- my rival plays Lucario. I beat him usually, but I still feel the matchup is in Lucario's favor- I just know it well.
For one thing, save yourself the trouble and don't go for the complex Dair combos. Save those for Snake or the space animals and heavyweights. Lucario's too slippery.
In the air, retreating C-sticked Fairs are your
best friend. Lucario's will usually try to get you in Fair combos. Retreating and fairing nukes this. If he does land a Fair, you can break the combo with a Nair sometimes. DI away from him and try to nair.
Toad works wonders, but you have to be cautious. Missing a toad can give him a free opportunity for a fully charged aura sphere.
Unfortunately, you won't be landing a lot of usmashes. One of Lucario's nicest kill moves is his Dair, and it outranges your usmash- he will usually land Dair if you're trying to usmash. However, Peach's utilt completely destroys his Dair!
If Lucario's coming in from above, utilt him. NOTE- if Lucario uses the dair EARLY so that your utilt MISSES, he can fast fall and dair you again before you can utilt him again. If you miss a utilt, SHIELD his second dair. Only a Lucario that knows the matchup will use the "dair early" trick. In fact, Shielding Lucario's Dairs is an excellent way to punish it.
Don't go for too many close range attacks like grabs, because Lucario's jabs are one of his best combo starters. However, Peach's jab-to-grab trick works pretty well against him
The trick to beating Lucario, however, is killing him early. At low %, Lucario has no killing power, and at high %, he becomes far more powerful. Kill him early on the first stock, and you will probably win the set since you can beat the crap out of him on the next stock before he gets to killing strength.
The instant you get Lucario offstage, rain turnips on him, then grab the ledge. If you land a turnip on him offstage, you will probably land a kill. Since his up-B does no damage and the turnip will mess up his second jump, you can safely just grab the ledge and hold it.
Also, Nair is an incredibly good punishment move on him. You can land it in between his Dairs, to punish any attack he lands on your shield, or to punish his fast up-B.
If you're offstage, watch out for his Bair. It can kill you.
Also, learn to deal with his Fsmash. Generally, once Lucario passes 100-120%, he will start spamming fsmash because it becomes really powerful at that point. It lasts deceptively long, and his lag disappears really quick so punishing it with a Fair takes really close timing and might not even be worth the risk.
When he has under 50% damage, be fearless and beat the crap out of him. When he passes 100%, be defensive but look for key moments to land kill moves.