Mewtwo has a massive advantage. Both in the Super Smash Bros series and in the official Pokemon franchise. Mewtwo's base stats are SIGNIFICANTLY higher than Lucario's, with the only stat even comparable between them being base attack. Mewtwo also has a massive movepool, even allowing him to learn sevaral moves capable of wiping Lucario from the face of the planet, including Fireblast, which alone would pretty much ensure a OHKO against Lucario. Meanwhile, Lucario has a relatively limited movepool typewise, and can learn hardly any moves that would have a super effective bonus against Mewtwo. In fact, the large majority of moves Lucario learns would put him at a disadvantage, due to most of them being Fighting type. Smash Bros wise, Mewtwo is still superior to Lucario in every way. Sure, Lucario might gain strength as he gains damage, however, this ability is quite the double-edged blade. In order for Lucario to be even the slightest threat, his damage percentage would need to be over 100%, at which point almost any of Mewtwo's unfairly powerful attacks would surely defeat him. Keep in mind, Mewtwo had the strongest throws in Melee, and he still has the record for strongest projectile (not counting PK Flash). Even with Lucario's aura sphere at max charge with his peak damage, it would still do less damage than Mewtwo's fully charged shadowball. Game over, Mewtwo wins.
Mewtwo got beat by Pichu , while Lucario has a chance against Meta-knight , like in that SSE mode and the improved Pikachu.
Hey , Mewtwo , you are not the most powerful Pokemon , you are a joke , a fake Mew , you weren't meant to exist like I was , Pichu beat you and you can yourself the most powerful Pokemon ever ? HAHAHAHAHA , even in Pokemon , Arceus is now the most Powerful Pokemon while these guys , now rival you , Also ,Did I forgot to mention that you are no longer the strongest because of Sucker Punch ? . I am a Pokemon who has reason to exist and to fight . *transforms into * , I AM THE AURA-GUIDING HERO , LUCARIO!!!
Lucario : You fool , you couldn't kill me , you just damaged me a lot , and you forgot the Zenkai ability to get stronger after recovering near death about my ability , I get stronger as I take damage , now I am stronger than before!! *rises into the sky and charges a stronger Aura Storm* , IF I'M GOING TO HELL , THEN I'M TAKING YOU WITH ME!! , THIS IS A SUPER AURA STORM , THE STAGE WILL BE DESTROYED WITH YOU AND ME!!
*the stage shakes*
Riolu : Don't do this Father , the whole stage shall be destroyed , NOOOOOOOOOOOO!!!
Lucario : I don't care !! Fires Super Aura Storm at Mewtwo , * most of the stage is destroyed, Mewtwo along with it*.
Lucario : Phew , looks like I shall always be in Smash Bros. , no one shall ever take my place .
Riolu : You , You , almost destroyed the whole stage ....
Mewtwo: *Uses teleport uses psichic stops Lucaraio from moving and throws him into his blast then teleports Lucario into the sun* Well thats bout it. You are now officially in hell. And your aura can't even help you.