If Deoxys has a Plasma Energy attached to it, can't it OHKO a Mewtwo EX that just attacked? It says 30 points plus another 30 per Energy attached to the Defending Pokémon. X-Ball requires at least two Energy to use, so that means 90 points of damage, then Weakness kicks in so that Mewtwo EX is OHKOed (10 HP left if it had PlusPower or Giant Cape equipped).
For that matter, Mewtwo EX needs extra Energy or Energy on Deoxys EX or a PlusPower to OHKO Deoxys EX. While those requirements are not hard to meet, if you do attach a third Energy now all someone has to do is drop Blend Energy GRPD, Prism Energy, or an actual Psychic Energy then use Colress Machine to attach a Plasma Energy from your deck so that another Deoxys EX can OHKO Mewtwo EX, even if it was completely healthy and had Eviolite attached.
Forgive me if I am being blind, but why are people looking down on the attack of Deoxys EX? I realize it doesn't get a boost from its own Ability (or from any other copies of itself), but if your deck can meet that (P) requirement (I assume most Plasma decks will run Colress Machine/Plasma Energy - I could be wrong) you've got an anti-Mewtwo EX attacker that also provides a +10 damage to your other Plasma Pokémon.
In fact, with a lot of the more Energy hungry cards, 30 + 30 per Energy (yes, I know it eats up a Plasma Energy) isn't exactly terrible. I wouldn't build a deck around this attack, but for what would otherwise be a Bench-sitting Basic Pokémon EX with 170 HP? Doesn't seem bad to me.