Cresselia-EX is an interesting card. It has a Psychic weakness, and it does 4 for 90. So what does it see a bit of play? The answer is simple: the effect of the attack, nullifying weakness.
Cresselia-EX is a Psychic Pokemon with 180 HP. This is the usual for a Pokemon-EX, and it is good. It is a Psychic type, meaning that it could KO a Mewtwo-EX. It has a bad Psychic weakness, and an OK one retreat cost. It's Ability heals 10 damage from it in between turns. So being weak to Psychic, why is it playable? Because it's attack, Psychic Protection, negates it's weakness, and does just enough to KO an un-Eviolited Mewtwo-EX.
Having the attack Psychic Protection makes it mostly used for only one thing: KOing Mewtwo-EXs. Having the attack makes it a great counter to Mewtwo, KOing it, and being unable to be KOed with another Mewtwo-EX, though they could just use another attacker. It can also be used to deal and sponge damage, because attackers like Darkrai would need to 3HKO it.
Strategies and game play:
When playing this card, you have to be wary of when exactly to place it down, because a fully powered Mewtwo-EX could OHKO it when it is Catchered. You have to either instantly attack when you place it down, or make sure all your opponent's Catchers have run out. You also have to be careful when it is on the bench, because another Mewtwo, DCE, and a Catcher can instantly KO it for 2 free prizes.
Combos with other cards:
The most obvious one is the cards in Darkrai/Hydregion. Move Energy to Cresselia-EX to attack, then give it free retreat with Darkrai-EX, and then Dark Trance and heal all the damage with Max Potion.
Gold Potion also combos with it. By healing off 90 damage, you can reset the damage, making your opponent to hit it another two time to KO it.
Other decks with energy acceleration also combo with it, though not as much. It could work, but cards like Mewtwo-EX work so much better, so it should be avoided.
Ways to counteract it:
Pokemon that can OHKO or deal at least 110 damage, like the Plasma Storm Black Kyurem-EX and Keldeo-EX could easily KO it.
Pokemon Catcher is another way. By dragging up Cresselia-EX, Mewtwo-EX could OHKO it with enough energy.
Finally, there's Sigilyph. If you have too hard of a time dealing with Cresselia-EX(which you shouldn't), you could always slowly drain it's HP, while it can't attack.
-KOs Mewtwo while Mewtwo fails to KO it when it attacks first
There is a regular and FA version of the card.
Modified: Overall, it is an OK Mewtwo-EX counter, and does OK with the rest of the field with enough Energy, though it needs a lot of them. 6/10
Limited: 90 damage is huge in limited, though it takes a while to power up, it can become a great healing tank, provided you can fit some Psychic Energy in. 8/10