Well I got Pokémon Platinum last week, and I finished it really quickly. I realised that for the last few times I've played these games I have done it in the same way, and it has been both easy and fast. For that reason, some people may be discouraged from using this method, but I don't think it takes anything out of the gameplay, and you can always build an awesome team after you have beaten the main game.
So here is what I do:
1. Use only the Pokémon chosen at the beginning of the game in battles. In most cases it is best to choose either the Fire or Water type Pokémon.
2. If you need to use HMs, catch another Pokémon to learn them, unless they are powerful moves such as Surf or Waterfall. On the newest games Bidoof is a great Pokémon for doing this as it can learn almost all HM moves.
3. Make sure your main Pokémon holds an item that boosts the power of the type of its strongest moves, or that ups its critical hit ratio. If the Pokémon is slow, you may want to give it an item to boost its speed.
If you do this your Pokémon should quickly become strong and should be able to defeat most other Pokémon in 1 - 2 hits, including the Elite Four's Pokémon. However, the way you play the game is up to you, you may want to just take it slowly and raise a whole team rather than one strong Pokémon.