Oh, Diddy is five times as strong as a human then? Well, that'd be god and all...
IF HE WAS ACTUALLY FIGHTING A HUMAN. But no, he's fighting a small yet powerful fighting machine that has taken down much more fearsome beasts, even without Aura. Diddy has taken down crocodiles with an I.Q. lower than their shoe size. Uh-hu, lucario can take down little animal pets and gods that aren't actualy gods that have a BIG chance of beating him if they were player controlled.
Also, Diddy is about three feet when translating to real life; he's a small chimpanzee in the games so why are we translating him bigger than he already is? Sure, Lucario isn't a tall fella, but Diddy is teeny. So what if he's stronger than the average human, so is Lucario? Moot point good sir. Still as big as Lucario, and still extremelly strong. Lucario is strong but not broken strong.
Also, Lucario is capable of many attacks, for many situations. Just because he has Aura doesn't mean he can't use other attacks that will hinder Diddy, such as:
Metal Sound- Can disorient Diddy and leave him defenseless. Unless he has ear protection, he will be affected too. No sound beams here. Stupid move.
Quick Attack- A swift move that will blindside Diddy without the speed of Extreme Speed.Diddy is ALWAYS fast, he does not require small speed burst attacks to evade or dissorient enemies. And he still has better movility.
Close Combat- A very powerful assault that will likely leave Diddy blodied, if not dead. If he can touch him. Diddy still is faster and has better movility than him and can hit him out of the attack or run while the attack is happening, dealing minimal damage.
Protect- A defensive move that does not require Aura, but can negate Diddy's attacks.[That leaves him incapable of attacking and moving and vulnerable for another attack. No turn based combat here.
Toxic- Can poison and weaken Diddy, likely killing him. Lucario does not have poison glands. Unless he can make poison in the middle of the fight, it won't happen.
Dig- Lucario can dig into the earth and pop out from unexpected places; Diddy won't be able to see him coming. Not as fast as in the anime. He will slowly dig his way to reach diddy and will be too tired to continue fighting once he gets out.
Double Team- Similar to Metal Sound, except Diddy could actually strike the correct Lucario. Even then, chances are slim. No creating replicas of yourself in the real life. Won't even work.
Endure- Can defend against Diddy's attacks if he is low on energy or is close to death, and could very well make a viable comeback. Sure, why not. Will still leave him bloodied, but alive I guess.
Iron Defense- Can make Lucario's skin practically unbreakable OR unbeatable, and requires no Aura. Turning you body into metal is not realistic.
Magnet Rise- Changes Lucario's ability to enable him to Levitate. Again, requires no Aura whatsoever. Is there a gigantic magnet above him that lets him levitate? Not realistic.
Cross Chop- The fighting type equivalent to Stone Edge, except its power is increased due to Lucario's fighting prowess. Sure, no prob.
Mind Reader- Allows Lucario to render Diddy completely predictable, and can lead to Diddy's defeat. Are there psychic powers inthe real world? He still could get detect, but not mind reader.
Reversal- Technically, an attack that does more damage the more hurt Lucario is. If, indeed, Diddy can beat Lucario to near death, Lucario could one-shot Diddy. Not realistic. HE WON'T EVEN BE ABLE TO MOVE IF HE IS THAT DAMAGED.
Sky Uppercut- Can knock Diddy Kong into the air for a follow-up attack once Diddy lands. Lucario is lighter than Diddy, he wouldn't even knock him on the ground. SCIENCE says so. And not sky high uppercuts.
-------------
As you can see, Lucario has plenty of attacks for his arsenal that require NO Aura whatsoever. In fact, the only things that could even REQUIRE Aura are Aura Sphere and Foresight.
Diddy is still dead. Which pains me because he is my Secondary.
EDIT: Again, IVYSAUR VS. DIDDY WOULD BE THE MORE EVEN MATCH-UP.