Arceus... you should pick a type first... does Dragon Arceus learn Outrage + Draco Meteor? Because that makes a really great double stab for it.
Uxie, you can use as a calm minder... Calm Mind, Grass Knot, Psychic, and Thunderbolt with a Life Orb... use it like a suicune or a Cresselia, but with a better movepool.
Mew, I'm not sure, but you could use it as a mixed attacker... like Heatran with special moves and explosion.
Kecleon works in trick room, or you could give it a priority set, since it really won't matter. Max Attack/HP or Special Attack.
Giratina can use an origin forme set... max attack and speed, then slap on a choice scarf or life orb...
Cresselia you can use either a calm minder in trick room OR a trick room setter.
Uxie, you can use as a calm minder... Calm Mind, Grass Knot, Psychic, and Thunderbolt with a Life Orb... use it like a suicune or a Cresselia, but with a better movepool.
Mew, I'm not sure, but you could use it as a mixed attacker... like Heatran with special moves and explosion.
Kecleon works in trick room, or you could give it a priority set, since it really won't matter. Max Attack/HP or Special Attack.
Giratina can use an origin forme set... max attack and speed, then slap on a choice scarf or life orb...
Cresselia you can use either a calm minder in trick room OR a trick room setter.