Mega mewtwo raid

I see a lot of people complain about how people just dont break the shields and i see people say that their mega just gets oneshot and my question is just how exactly does this happen? Like my metagross at level 37, 11 defence and 10 hp (ivs) is able to tank an non-dodged flamethrower from mewtwo and only does half his health and a bit less than a quarter if it is dodged so this bring me back to the beginning how are people getting one shot as soon as they send in their mega?