I noticed in prereleases that Deoxys... REALLY dislikes having copies of itself in your hand. Sure, I was playing with only two copies at most of a Deoxys, but still, it seems pretty frustrating, even WITH Cynthia's Feelings.
I think Giratina from the IFDS Starter Decks would love to be paired with Deoxys. It's power forces both players to shuffle their hands into their decks and draw 4 cards whenever its played. Other then disruption, I think damage spread would be a good idea to pair with Deoxys. Defence Forme's attack is pretty nice, just it needs something efficient to play damage first. Speed Forme's good at finishing off stuff, but not very good at setting up KOs, I don't think (only 30 a turn's a tad slow... Compared to Empoleon and Lucario DP's 60 a turn, the highly efficient spread of Yanmega, etc.). Attack Forme's attack... I don't particularly care for the three energy cost, AND it can't really use that attack in succession, and Normal Forme requires the opponent to be set up.
Yanmega sounds like a decent start though, especially since it won't draw away from the energies that Deoxys will be craving.