I've also thought of teaming Mewtwo EX with Gardevoir (Psychic Mirage). Gardy's Abilty doubles your psychic energy in play; So, you could X-ball attack with only 1 psychic energy.
There is also the old "Psy Shadow"
Gardevoir that was released in
Ruby/Sapphire as well as
Power Keepers; its Poké-Power allows you to (once per turn) search your deck for a basic
Psychic Energy card and attach it to one of your Pokémon in play. The bad news is that it places two damage counters on that Pokémon. The good news is that we are talking about Unlimited for this deck so odds are, you can deal with that. Unless they addressed it when I wasn't looking,
Brock's Ninetales would allow you to have a "fifth"
Gardevoir in play. Why do that? Well the current
Gardevoir with Psychic Mirage should still have access to its Ability, because Pokémon Power =/= Ability as far as game mechanics are concerned.
So... while it is a ludicrous amount of cards, you could use
Broken Time-Space to get all of these into play in a single turn, manually attach one basic
Psychic Energy card to
Mewtwo EX from hand, then another four from the deck via Psy Shadow, drop some healing for the 8 damage counters (
not Max Potion =P), and
Psychic Mirage would double all of that so that
Mewtwo EX had an effective 10 Psychic Energy attached!
Of course it would be much better in the end to just back
Mewtwo EX with
Slowking (from
Neo Genesis) if you were going to risk any smaller Pokémon in the deck; no where near as creative or fun, but brutally effective.