ADHD pretty much said the truth, to be honest I'm a crappy Lucario. The only time I've made money in this game is when I went teams with Sago who was the best player there. I'm working my arse off to get better with Lucario and Link but despite this I can at least analyze traits well enough to give input. Enough of my stupid emo rant, here's my input.
Lucario needs to keep close to Diddy when he's trying to draw Banana's. Pressure is key here, stay on him and don't let up, how you play will change if he gets a banana. If he doesn't have one, stay on him and don't let up. Fsmash is safer if he doesn't have a banana and you don't spam it. Tilts are more effective to space pressure.
Lets say he does toss a banana to make a wall between him and the banana, my advice is to learn to Z-catch and Z-drop. I've found it easier for myself seeing as the bomb shenanigans I learn with Link and a few turnip things with Peach. If M2K can Z-Catch & SHfair immediately afterwords I don't see how Lucario can't Z-catch and still apply pressure. It's easier to deal with bananas if you can do these on reaction, and fire AS if you predict a glide toss, the MU is easier to deal with.
Now if he has a banana and one in front of him things change. One thing you could do is wait out for one banana to disappear or get close to it and try and approach from above. Otherwise your going to have to approach carefully, Diddy out-camps Lucario there is no debating this. Your going to have to approach to make sure he doesn't keep pulling Banana's and to well win.
For killing Lucario will kill Diddy much earlier, and his trouble killing doesn't help him. He does have ways to set-up kills, something Lucario has a few problems in this MU doing.
So one mistake Diddy could Die, One mistake Lucario could be set-up for a kill, it could go both ways, just on character kills earlier.
Stages, ban FD, never ever ever ever go their. When I fought my first Diddy ever I got ***** their from not knowing how good the stage was for him. I did much better on Frigate and Yoshi's Island.
I still think this is 55:45 Lucario and is his easiest S-tier MU. I know this MU is heatedly debated right now, but I'll leave my thoughts on it anyways.