It's got some nice HP, a decent first attack, a irrelevant weakness and a awful retreat cost. The second attack is of course the reason to play the card. It's retreat cost is too heavy to be retreated. The cards RogueChomp mentioned (Keldeo EX, Blastoise) work well, of course. But I think a deck with Mew EX and Skyarrow bridge could be interesting. Set up 2 Mews and a Walrein (And probably a Blastoise) and keep retreating the Mews into each other to negate the effect of Ice Entomb.
Because of Mews awful HP, once you have the kill on something, you send up something less fragile to KO the opponents Pokemon, hopefully that can work well with Skyarrow as well. The Keldeo Promo seems like a good one. Walrein/Mew/Blastiose/Keldeo might become one of the kings of the fun league decks. But probably nothing more