I don't have enough Mewtwos to test this out, but could this help you if your meta is a bunch of mewtwo decks? Since Mewtwo vs Mewtwo pretty much depends on who goes first..what if you had this Leavanny on your bench? So that way you can't be knocked out in one turn.
With out Leavanny:
Opponent has 2 energies on Mewtwo EX (active)
You have 2 energies on Mewtwo EX (active)
Opponent goes before you do, does the first attack. 4x2=8 x2 because of weakness = 160 (+ plays a Plus Power) = 170dmg KO
With Leavanny:
Same every thing except it eliminates weakness, so only doing 80 dmg(90 at most w/Plus Power) that turn. Gives you the chance on your turn to KO them, or at least survive some hits from Mewtwo or Chandelure.