Well, I see a PlusPower there in the discard. If that was an attached energy, it would have done 120 more damage (30 more x4 Weakness).
Let's see (X-Ball)...
My side setup:
Active: Mewtwo EX - 53 Energy (possible via method described in previous post)
Bench: 1-1-1 Gardevoir
Discard: 2 Mewtwo EX
Opponent's side:
Active: 1-1-1 Gardevoir - 48 Energy (60 - 9 Pokemon - 2 Prizes - 1 Hooligans)
Bench: 1-1-1 Vileplume
Discard: 2 EX Pokemon and 1 Non-EX Basic (so that I took 5 prizes off as few Pokemon as possible)
X-Ball for ((54+48)x2x20)x4=16160
Questions to look in to:
Maybe 2 Hooligans are realistically needed to limit your hand to 0 after drawing your fifth prize? This takes one more energy off their side, which means -160 damage to make the total damage 16000.
Things I considered, but found made less damage:
Revive - not put in because then you have to pay the retreat cost for Gardevoir, which lowers your energy. You could then run a Super Rod or something, but that takes away more energy cards. You could be thinking "Why not Super Rod the Mewtwo and the retreat cost energy?" - without Mewtwo on the bench, Garde can't retreat, so the retreat cost energy isn't in the discard.
Mew on your side, Swanna and Mewtwo somewhere on field - uses 3 cards for +40 base damage. That is better spent with 3 energy, which is +120 base damage because of Gardevoir's Ability.
________
Let's see (Dragon Burst)...
My Side:
Active Mew EX (1 lightning, 57 fire)
Opponent's Side:
Active: Something weak to psychic
Bench: Rayquaza EX, Vileplume, Swanna, something to copy that doesn't discard energy
Discard: 5 Prizes worth of stuff
(57x60+40)x4 = 13840
This situation is more likely because it doesn't depend as much on what is prized for each person. There are also more card possibilities for your opponent since the damage is independent of their energy. If you replace the Lighning Energy with a Prism, your opponent doesn't even need a fourth benched Pokemon - you can attack with Vileplume or Swanna's attacks to take 5 prizes.