Roceus@ Stone Plate
Timid
100 EVs to everything, 8 to Attack.
Calm Mind
Judgment
Earth Power
Recover
*Using this in Sandstorm really allows Roceus to Rock, as the SpD boost works with Calm Mind, and Judgement's 180 base Rock move dents Shaymin-S, Ho-oh, Lugia, Rayquaza, Salamence, Abamasnow, while Earth Power covers Dialga, Metagross, Jirachi, Heatran, etc.
With Quak-Edge (Ground/Rock) coverage in the form of Special Attack, and Judgement dealing about the same as Ice Beam would to Ray, Mence, Shaymin-S, and even more to Ho-oh & Lugia, this Arceus can dish out the damage.
Plus, it makes a great Ho-oh counter as it resists its STABs, without fearing Burn (Special Attacker).
On a sandstorm team with Tyranitar, Arceus, & Garchomp, you could really take advantage of the boosts the weather brings.
Also, Groudon could be used to remove the Water weakness leaving Roceus weak to Grass, Fighting, Steel, Ground, which, ironically makes Ho-oh a perfect partner, being able to switch into all of those moves with impunity; Ho-oh also benefits from Groudon's sun, due to removing its Water weakness and boosting its Fire attacks. Additionally, giving Roceus Flamethrower/Fire Blast over Recover would make Roceus able to KO Steel & Fighting types like Lucario, Heracross, Toxicroak, Forretress, Scizor, while gaining a boost to its Fire move.