You can't really go further then that. The pokemon league contains the toughest trainers in pokemon platinum.
The Sinnoh League Champion "Cynthia" has a Spiritomb.
just keep practising with the Pokemon league until your Pokemon goes to level 100. Or If you haven't got to the league yet you can keep battling trainers and wild Pokemon.
The Pokemon league is where after you win 8 badges, you get to battle the elite 4 (four tough trainers) and the champion of the region (in diamond it's Cynthia until you beat her).
No trainers have it, but to see it, go to Backlots study after beating the League and press A in front of the book on the table to see it. OK?
It's in the same place as in Pokemon red, blue,yellow. You go to New bark Town swim in the water to the right of your town. Then you fight some trainers and find a cave. then your at the Pokemon league.
If you are talking about Sinnoh, you find them at the restaurant at Lake Valor or at the Pokemon League!
none just beat all the trainers
You have to beat the Pokemon league in Hoenn, Kanto and Johto. In Sinnoh, there might be some trainers who have Torterra.
The Pokemon league is a team of four trainers that are the best in the game (not including you) you have to get through victory road to get to them. They are located north of sunnyshore city. WARNING when fighting them, you cannot take a break to to to a Pokemon center, so stock up on revives and hp restores and pp restores. (If you have AR some of this may not apply.)
To get to the Pokemon League in Firered/Leafgreen. You must have all 8 gym badges, have a team that knows all HM's(you'll need them), and a strong team to get past the trainers. Best of luck. Pokemon Master Chewie.
In pearl, you start going to the Pokemon league. Battle ALL OF THE TRAINERS on that route and one of them has it. hope this helps