I dunno what you're trying to say with that. First of all, that's not Ash, that's Red. Second of all, His Charizard didn't somehow magically one-shot Blastoise with a single Seismic Toss. He landed 3 moves, and only ate one Hydro Pump. That's a much more plausible scenario.
Eeeeh not really.
WARNING INCOMING POKEMON NERD MATH BECAUSE I HAVE NOTHING BETTER TO DO
We're assuming Red's Charizard is a mixed offensive set, considering he used Mega Punch, Fire Spin, and Fire Blast. We're going to be generous with our damage calculations, and assume Charizard is running max 252 investments in both special attack and attack. We're doing this to give Charizard the maximum potential to actually KO Blastoise with the given moves.
For Blastoise's spread we again want to be as generous as possible for Charizard. We'll give him a spread of 252 attack, 84 special attack, and 172 speed. This spread was chosen for a couple reasons. One, it has absolutely zero defensive investment. We're trying to absolutely maximize Charizard's chances so we'll ignore the fact that Blastoise is a traditionally defensive Pokemon. The 172 speed investment is important because it puts Blastoise just 1 speed shy of outspeeding Charizard, this ensures Charizard goes first and that the fact that Charizard got to use three moves to Blastoise's two continues to make sense. Blastoise's third move would have come after the Fire Blast, but he was KO'd before having the chance. The 252 attack is invested because in this case, he never hits Charizard with a physical move and as such it's a "throwaway" stat in this scenario. The rest of his points that he can invest go into special attack; putting any more points into speed will have Blastoise outspeed Charizard and this whole scenario could no longer be true, and put the points into any sort of defensive stat makes things even less likely for Charizard. So with this setup, Blastoise would still have an about 19% chance to 0HKO Charizard with the Hydro Pump. Obviously Green just wasn't quite this lucky with the damage roll.
We'll give Blastoise a Naughty nature. This raises his attack and lowers his special defense. This is good because it gives Charizard even better odds of dealing enough damage, and the attack increase doesn't have an impact. Charizard has a Mild nature which boosts his special attack and reduces his defense. Since he isn't hit by a physical move the defense reduction doesn't matter, and the special attack boost has the best odds of him prevailing.
The Mega Punch he starts off with will do anywhere from 22-26% to Blastoise, and we'll assume max damage roll with the 26%. Charizard eats the Hydro Pump, and Charizard clings on with a sliver health; because he's at such low health his ability Blaze activates which boosts the power of his fire moves. He gets him with a Fire Spin, which is another 15-18% thanks to Blaze's powerboost and of course we'll take the higher. Looks like Blastoise dispels Fire Spin on his turn, we'll assume a Rapid Spin or whatever, and then he eats a Fire Blast. The Blaze boosted Fire Blast deals 47-55%, and we'll take the higher again. So even with max damage rolls... He comes up to 99%. So the only way he wins... is that he scores a lucky crit.
But as soon as you give Blastoise an actually good setup, like 252 HP and 252 defense with a Bold nature the only possible way Charizard wins in this scenario is with EVERY move he used critting, AND with two of them being no less than max damage rolls.