First, I'll assume that you just want to finish the main quest in Platinum. Please tell me if this assumption is wrong, and I'll convert this to a comment.
If this is for the main quest, giga drain and leaf storm on a Torterra are both worse than wood hammer, so don't use anything that raises its special attack. If you decide to use curse, speed will also be useless, so go with a physical attack raising nature like adamant, lonely, or brave. If you decide to use swords dance, your Torterra will be able to raise its physical attack, but its speed will stay in the range that lets it outspeed some of the opponents. A speed raising nature, like hasty or jolly, would be especially useful on this kind of moveset.
Regardless of which nature you have, experience and levels will be more effective than natures, so it's probably a good idea to battle every trainer when you enter each area for the first time.
Torterra @ miracle seed/earth plate/expert belt
EVs: 100 Atk/100 Spe
- curse/swords dance
- earthquake
- wood hammer
- synthesis/stone edge