Here's my suggestion:
Latios @ Salac Berry
- Dragon Claw/Ice Beam
- Psychic/Thunderbolt
- Calm Mind
- Recover
If I remember correctly, Quiet Nature boosts your Sp.Atk (which is good) but drops your Speed (not so good,) so Salac Berry for the item should be able to lessen the blow of having reduced Speed somewhat. Calm Mind up until you're about to die, then once Salac triggers (assuming it does,) Recover all your lost HP back and then start murdering stuff. For your attack moves, pick any two out of the four and see what you like best - Psychic/Dragon Claw gets double STAB but is also walled by Steels, Ice Beam/Thunderbolt gets immense coverage but no STAB at all. Those are just two examples of what you can do.