First off,
Hi, Smooth Criminal. Long time no see. I've been good.
Here's a list of moves that Lucario could do without being all supernatural and crap.
Quick Attack - Self explanitory
Foresight - This just makes Lucario able to hit 'evasive' foes, so if Diddy's jumping around and crap, it will make it easier for Lucario to hit him
Detect - We went over this
Metal Claw - Spikes on his hands
Counter - Counterattacks with double the force of the attack Lucario was just hit with
Swords Dance - This is just a little dance thing that somehow makes Lucario hit harder
Focus Punch - It's just a hard punch
Calm Mind - "The user quietly focuses its mind and calms its spirit to raise its Sp. Atk and Sp. Def stats." Nothing special, you're just calming down to keep focused
Roar - Lucario roars, to scare the crap out of Diddy Kong. This has the disadvantage of giving Diddy smelly ammo, so...
Bulk Up - "The user tenses its muscles to strenghten its body, raising Attack and Defense." Nothing wrong with that
Frustration - This is basically just an attack that gets stronger the more pissed off you are. Judging by my own experiences playing against Diddy Kong, this will be a good move to have.
Iron Tail - "The foe is slammed with a steel-hard tail." Now, forgive me, but isn't Lucario steel type? What's with all of this 'only his spikes are steel' crap I've been hearing? Wouldn't it make sense that if his spikes are as hard as steel, his skeletal structure would be as well? (Seeing as they're both Lucario's bones)
Brick Break - Nothing special, I watched some Korean man use Brick Break on TV yesterday
Attract - No, I'm kidding. Making sure you're all paying attention
Bone Rush -
I see NO reason why Lucario shouldn't be allowed a held item. In which case, give him the Thick Club, which is basically a big bone club.
Fling - See above
Payback - If Lucario uses this after Diddy hits him, it does alot of damage
Swagger - This confuses Diddy Kong, which I presume translates into him fighting recklessly (and thus missing sometimes). However, it also makes Diddy stronger, so I don't think it'd be a great option
Strength - Lucario hits Diddy Kong with an attack that's strong enough to push huge boulders. Owch?
Rock Smash/Climb - Meh...
Agility - Nice move, makes him all 'float like a Butterfree, sting like a Beedril'
Bite/Crunch - Lucario's got some nasty teeth on him
Bullet Punch - Just some really fast punches
Cross Chop - Just a chop
Hi Jump Kick - Might not be the best choice, but it could work
Low Kick - People keep saying Diddy Kong is heavy, so this is a good addition. It does damage relative to how heavy the opponent is.
Sky Uppercut - Just a really strong uppercut move
Mud Slap - This is much more useful realistically than it is in the game, because here Lucario kicks some mud into Diddy Kong's eyes and then he's pretty much blinded
Zen Headbutt - Lucario = Steel Type = Owch
Moving on from that...
What's with arguement about since Lucario's build is slim, he's weak or something?
He's a video game character, their build means nothing. Look at Machop!
Its muscles aren't very defined, but the little guy knows every martial art in the world, and builds its strength by tossing around huge Gravelers all day.
If you want a less extreme example, look at Sonic. The dude has string-bean arms and he's still stronger than most humans. I'm not going to use Shadow, who can flip over buses with one hand, as an example because he's genetically enginered.
Lucario is not 'frail', either. Does it have an average defense? Yeah, I won't lie. In fact, its tied for the lowest base defensive stat of all Steel Pokemon (at 70). Who is it tied with? Magnemite.
If Stats are any indication, than Lucario's body is as hard as the metal creature shown. Frankly, it's kinda hard to dent.
And Lucario's physique is just wierd as hell.
It's torso and above is fairly obvious (jackal-like, with a blindfold for the lulz). It's legs are just wierd though; its thighs look like shorts for some reason, and it has the feet of a Kangaroo. I dunno. I would guess that Lucario has either a very hard skeletal structure or very tough skin for him to be classified as a Steel-Type.