If you have Lysandre handy.
If your opponent has an appropriate target on the Bench.
By this logic though, other decks do not have to take down M Mewtwo-EX, just its Bench.
This point is not entirely without merit, but it isn't the be all end all answer either. As a hypothetical, because I don't know M Scizor-EX decks except as a general concept, can they try a trick like shifting gears to Scizor-EX as well? I mean, are you going to keep using Damage Change while Scizor-EX puts just enough damage on you to set up for a 2HKO the next turn, but not enough that you'll get to Lysandre up something from the Bench and Damage Change for a OHKO? Really asking because I don't know. ^^'
Well, this thread reminds me why folks are so excited about Mewtwo (XY: Evolutions).
I have a m Mewtwo EX/ Garbodor and played against Scizor EX a few times and they all ended the same way. I was just able to damage change and place damage on other Pokemon. Mewtwo hits them too fast and hard and while saying just hit the Shaymins is hypothetical, it is a factor. For me, its been 4 free prize cards, between the Shaymin EX and Hoopa EX, meanng in all my games, I just needed to KO a single Scizor EX. Taking 2 prize cards while I completely heal m Mewtwo EX is a big deal. Its hard to say right now because no one really knows how to build Scizor EX but my build runs heavy Max Elixir and Mega Turbo so if they manage to discard my basic energy, I can still get an explosive turn.
Another thing for the match up is you don't even need m Mewtwo EX. You can go solo mewtwo to lock in your Damage Change and then mega evolve when you need to KO something so Mewtwo EX is capable of just playing around Scizor EX because Scizor can't OHKO you.