As for Mewtwo ex itself, its X Ball attack deals 20 damage per Energy card between both Mewtwo ex and the opposing Active Pokmonallowing it to lay out some truly devastating blows even against some of the bulkiest opponents
It would be totally inefficient, though
A player can have any number of pokemon stored away, and only three in the game
For instance, if Endymion will be Summoned in any manner this turn, Souls can send Reflection to be revived by Selene later and then overlain with the Summoned Endymion to start the Vortex Combo (the same works if Endymion and Reflection are switched in this scenario)
One of these (Beta Edition) was sold for $13,600 USD on October 27th, 2020
This particular copy, graded BGS GEM MINT 9.5, is among the highest-graded examples, featuring sub-grades of 9.5 for centering, edges, and surface, and 9 for corners