The mewtwo deck is nowhere near invulnerable, in fact, it has
a critical weakness: the fact that it has only 1 or 2 pokemon in
it.
So, how to capitalize on this?
The trick is simply that they can only have 1 pokemon powered
up (all the energy must go to mewtwo) and its always not gonna have more than 1
energy. You might ask what if they use an attack, or don't barrier?
You must always have a pokemon out that can attack to prevent this, and attack
early and often. Second, to finish it off, you can use an energy removal
when it has but 1 energy on it, so it won't be able to attack the next
turn. However, only use this if you have a pokemon that can deal at least
enough to kill the mewtwo in one shot, and more damage is better. (beware evil
defenders/potions) Also, pokemon that remove energy can also work wonders,
once you hit it with the initial energy removal to start hitting it. If
they are foolish enough to place another mewtwo, then a gust of wind can bring
it into the line of fire (barrier only affects the mewtwo card that used it, so
even if another mewtwo switches out, its not affected) magneton and
magnemite can also deal damage to a benched mewtwo, should your opponent be
foolish enough to place one. If they don't? Then be sure to remove
energy so they can't barrier the next turn, then stomp them with a powerful
attack.
The other thing is, remember that if your opponent has no
pokemon in their starting hand, you MAY draw UP TO 2 cards, and they MUST show
you their hand. So, if your opponents hand is all psychic energy, or
perhaps there's a trainer (think if that trainer would work in a mewtwo deck)
then don't draw any cards (unless your starting hand rrreeeaaallllllyyy sucks)
and if their first (and only) pokemon is a mewtwo, don't use any cards that let
you draw. And also be sure to then use any imp oaks to make them gain
cards (buy you more time)
To conclude, be careful when drawing extra cards at the start,
and be certain to include at least 1 energy removal, if not 2 (if your playing a
best outta 3 match, then even if the removal gets into your prizes once, it
shouldn't happen twice) And, should you see a mewtwo, waste no time
pummeling it. Use any attacks and any speed that you can, to deal as much
damage as possible, so it will be easier to take out. And never let your
foe do anything but barrier, until you crush the
mewtwo's.