Heracross IS weak to psychic, but Heracross would have hit first and KO'ed mew before It had a chance to strike.
Porygon - Z is evolved from Porygon 2, it's a special attacker.
I have 2 mews now, one shiny and one normal, I've managed to get all the legends now (except Arceus because that hasn't been released yet...)
Oh and Thunder is a GREAT move on Kyogre because Thunder is 100% accurate when it's raining and Kyogre makes it rain.