I was battling a person online and he was using a Mega Kangaskahn. He used Power-Up Punch, which, for the mega evolution, hit twice and knocked out my Tyranitar and also raised his Mega Kangaskahn's attack by two stages. Then, I sent out my Mega Garchomp which had 252 EVs in HP ,making it quite bulky right? But when his Mega Kangaskahn used Return, my Mega Garchomp instantly fainted! He didn't even have to hit me twice! Is there an explanation for this, or is this normal?
Here's the damage Calcs: +2 252+ Atk Parental Bond Mega Kangaskhan Return vs. 252 HP / 0 Def Mega Garchomp: 472-558 (112.3 - 132.8%) -- guaranteed OHKO
This means that Garchomp's maximum HP was 420 (max IVs and EVs) and the damage that a Kangaskhan will do to it should be a minimum of 472, which is a clean knock out.
Hope I helped!