I'd say that Aegislash is a reliable counter against mewtwo with correct prediction. With kings shield, he could block an attack, and if he can set up a swords dance/ use a choice band he can probably KO.( I can't download a dmg calculator can someone check that?).
Additionally, it's not that probable that that king's shield is used twice in a row because of the need of switching into shield form after attacking.
If you're truly desperate for a full counter, you could always use a focus sash to score 2 shadow sneaks on mewtwo.