In the realm of Smash Brothers, it's possible, 'cause that's just how it works.
In a game sense, canonically, when we consider who's the strongest, we also have to consider the evidence we have. Mewtwo can't be controlled by his creators and science, but Mewtwo can be conquered by a ten year old. You see where there's a problem here? The games want you to think the Pokemon are awesome and amazing because awesome and amazing sells. People love Charizard and rightfully so. If we take his Pokedex entry at face value, he can produce flames so hot it MELTS SOLID ROCK. That's amazing, and the funny thing is that's just gravy on top of the delicious potatoes, 'cause Charizard looks friggin' awesome.
Having said that, very few Pokemon actually live up to their Pokedex entries. If Hitmonchan can punch at bullet train speeds, it should be damn near impossible to actually beat one. 310 MPH punches are NOTHING to sneeze at. What if we consider Mach Punch? A single Mach is 770 MPH. A Pokemon with Mach Punch breaks the sound barrier. Anything with Mach Punch should feasibly ANNIHILATE anything it hits, as well as itself. The kind of force it would put on itself going from 0 to 770 MPH in seconds should feasibly tear a Pokemon apart, and whoever it happens to hit, if it even gets close enough to hit it. While we're on the topic of Mach Punch and Mach Speed, why don't more people use Pidgeot?
Pidgeot flies at Mach 2. That's 1,540 MPH.
How about Heracross?
Heracross weighs 119 lbs., so it can lift 119,000 lbs, or about 60 tons. It can almost single-handedly lift the space shuttle. Forget using enormous moving equipment to move that space shuttle, guys, just grab a couple Heracross!
But wait! There's more!
Chesnaught:
It can flip a fifty ton tank? It can literally lift and flip a fifty ton tank? That's the equivalent of lifting and flipping
FIFTY GROUDON ALL AT ONCE.
Gardevoir:
JESUS H. CHRIST ON A CRACKER, GARDEVOIR MAKES
BLACK HOLES IF ITS TRAINER IS IN DANGER!?
Not convinced that Pokedex entries are 100% fabricated bullcrap? Here we goooooooo!
Pyroar:
Yes, Pyroar generates fire breath that's
ALMOST THREE TIMES MORE POWERFUL than Charizard's!
PYROAR GENERATES HEAT EQUAL TO THE SURFACE OF THE FRIGGIN' SUN!
"Gosh, ChunkyBeef," you say. "These Pokemon are extremely powerful! But what about the serene, graceful Water-types! Surely they can't kill or maim me!"
WRONG!
Dragalge:
Dragalge has a poison it can spit at you that can dissolve you in minutes if it gets on you. Worse, it shoots it
INDISCRIMINATELY at
ANYTHING IT DAMN WELL PLEASES.
Oh, man, but let's give Dragalge the benefit of the doubt! Surely the waters in a place like Kalos aren't that bad, right?
Clawitzer:
So, it's enough to essentially put a hole into you if it wants to.
In conclusion, all of the Pokedex entries are fabricated. They're fabricated
BY CHILDREN. Worse, there's evidence they're fabricated
BY YOUR CHARACTER, THE PROTAGONIST. You're catching Pokemon
NO-ONE ELSE HAS EVER CAUGHT BEFORE, SOMETIMES, HAVEN'T EVEN SEEN BEFORE. Your protagonist is literally
MAKING UP EVERYTHING ABOUT THESE POKEMON.
Therefore, all Pokemon's powers are, for the most part, grossly exaggerated and, frankly, the more I look into the topic of Pokemon's canonical powers the more I'm convinced that Mewtwo could get whipped by Ness and Lucas, two psionics whose powers aren't grossly exaggerated and we have actual legitimate proof of their power.
Yeah, the Pokeball is some sophisticated technology, not especially if they can make different kinds to suit different levels of income that have different and varying levels of effectiveness.
There's every bit of evidence, on the other hand, against these Legendary Pokemon wanting you to fight and catch them. Giving you the benefit of the doubt, if they want to fight they most certainly don't want to be caught, because I'm not sure if you've ever tried catching some of those legendaries, but they are FFFFFAAARRRR less willing to be caught than the average Pokemon. There's a reason why they have astronomical catch rates. No Pokemon ever seems to WANT to be caught. That's silly anime/movie stuff.