You know that if Mewtwo didn't terastallize, Tera storm wouldn't be super effective, and mewtow could simply live one hit and KO the next turn.( For those who are confused, the stellar type is super effective against a pokemon that is terastallized. And as you can see, Terapagos became stellar type and Tera storm was super effective.) an alternative way to win this is to literally slap a life orb into Mewtwo, and use tera fighting instead for stellar (which mewtwo isn't exactly the best user for tera stellar). Also, you should probably use Lugia... you know... for defence, not offence. There is a reason that thing is called The Great Wall.