Natures are easy. If you're going for a special sweeper you want one that raises either speed or special attack. And since you wouldn't be using the attack stat at all, lower attack.
In other words, Timid (+speed), Modest (+spatk) or if you want to boost spdef, Calm.
For moves, Surf and Ice Beam is a standard attack combo. Recover can be useful (although, if defense is low it might not be), then Hypnosis is a good option too, if you can take a few hits.