Use your Dash attack to lead into an up smash, juggle a bit with upair, then finish with a forward air or two.
Keep yourself spaced, if you see that upsmash coming, get the 'naners out.
Only use UpB if Lucas is spamming downtilt.
Use side B when you hear the PK of PK fire. Make sure you kick.
Keep this to a minimum, but, side B, up jump, immediately spike, 'cause most likely, he'll jump and try to up air you.
Lucas is one of the few characters you'll do fine spamming peanuts a bit against.
Rarely will a lucas run at you, so if he jumps, get your side air ready.
If you get a chance to knock him off of the stage with a side smash, feel free to jump, forward air, jump, forward air, then WAIT FOR HIM TO TRY AND COME BACK (If he uses PK thunder, quickly up B so you can gtfo and he'll be immobile and fall to death.) then use up B.
I know all of this because I main Lucas, and I use Diddy against my friend that uses Lucas a lot.