I recommend training them to level 48-50 since that's my Pokemon's level at the eighth gym while playing platinum. You should beat every trainer you find. I feel like you didn't battle most of the Trainers in the Game, which is why your Pokemon is so underleveled. Even if you did that, you also need more training for your Pokemon. I think a great spot to train them would be Iron Island, also use an Exp Share. But that would take a lot of time. You should provide your current moveset so that we can know your team's weakness.