Diddy does use a flamethrower in DK: Jungle Climber though. Fire is super effective against Lucario, but yeah... Lucaro does learn lots of moves.
Guitar Gazump is still a nice attack, and Pit won from Lucario using range advantage right? Maybe Diddy has a chance with Orange Grenades and the flamethrower. He also has invincibility for about 10 seconds, which he could use easily, cause in DK: King of Swing you just become invincible by pressing A (since you pretty much play the game with L & R).
Diddy has 300 bananas. He can use them for healing purposes which cost 10 bananas for 33% (or 1 heart, since he has got 3 hearts in total) and 20 for invincibility. 300 / 20 = ... 15 uses of invincibility. Each use of invincibility lasts for about 10 seconds, and DK and Diddy can just climb into enemies and they'll die. Attacks become more effective as well. (Which would be a rolling attack similar to the ones in DKC, but except these attacks take down huge ships and robots and ****. O_O No lie.)
There's a chance Diddy can win this. But it all depends on how long Lucario can last. He does learn Reversal, Endure and Subsitute amongs other things, but Lucario is more of an attacker than a wall anyways.
I'm looking at Smogon.com here which says that Lucario with a single Swords Dance can almost kill or do great damage against great defensive "wall" Pokemon as Skarmory, Hippowdon, Donphan and Forretress with Close Combat. Those Pokemon are deffinatly more durable than Diddy Kong is unfortunatly, so it all comes down to Diddy's speed, strenght and Lucario's overall defensive options.
Diddy may lose this one, but Donkey Kong at least will win this one knowing all this. Diddy still makes a pretty good chance I say myself. But then again, that's just ME.