If he spams his dair, just upB > dsmash > fsmash him.
It'll teach him.
Sometimes his dair goes through UpB. I play a Lucario regularly, so I've seen it happen a lot.
Of course, the upside to this is that dair slows his descent, which gives you more time to work up a counterattack.
The hardest issue of Lucario is his hitboxes. They don't hit anywhere near where they're supposed to. Naturally, his Fsmash has a huge hitboxes, but you'll see some strange effects from all of his attacks.
The most important point is to use Plasma Whip and Plasma Wire to keep him out of range, and fire a lot of Paralyzer rounds to try to create an opening. You can combo him pretty well up to high percents.
Then, the real trouble starts. Lucario has been, for me, very difficult to kill. Your primary concern will be getting off kill moves without setting yourself for punishment, because at high percent, Lucario will punish you hard. Avoid Flip Jump as a kill move, because it leaves you wide open, and try to keep it to bair and uair. If you can get off a lucky Plasma Whip or two, that's a good idea as well. Note, however, that at high percentages, dthrow -> uair does not work, so you'll have to find another way.
