Make sure you don't have too many Pokémon in your party with the same vulnerability. If you have a flying type, a grass type and a rock type.. well, you're pretty much !@#$% when you're up against Ice Pokémon. Having a Meganium, a Fearow and a Golem this is going to be one tough call for me tomorrow when I'm surfing down Route 27.
With the original Gold & Silver games, I remember beating the Elite Four with Pokémon of about level 43-44 without too much difficulty. Sure, I had to use a shitload of revives and hyper potions and had to reset my GB a couple of times (especially on Lance), but having a balanced team is a MAJOR help when taking on the Pokémon League.
For now: if all fails, I could always transfer my level 100 event Arceus from Platinum and steamroll the Elite Four without too much hassle. I just need a couple of PP-restoring items then.