Yeah that is true. But Mewtwo is very OP. Just hope they will actually damage Mewtwo
Yeah, between all the 151 original Pokemon. But a lot of Generations have come since then, Legendaries which power makes Mewtwo's almost insignificant (like Arceus), and even new types like Dark. I no longer see Mewtwo as powerful as he used to be back in Gen I.
Being realistic, Ash and Goh have no current Pokemon capable to resist against Mewtwo in a 1 on 1. Lucario and Cindereace are strong, but they are still "rookies". The closest thing Goh has to match Mewtwo's power could be his giant Golurk, and between all of Ash's Pokemon, the only one I see capable to fight or even beat Mewtwo would be Greninja (or maybe Melmetal). Of course, he would need a Dark Type move like Night Slash. So, the only way for Ash to have Mewtwo on his team in this episode would be if Mewtwo decides to go with him willingly.