I usually train before the poke league by just keep trying at the elite 4. U gain good EXP and get a lot of $ for items and such. ANd ya never know, you might beat it. I recommend lvl 55-60 if you want to go on a sweep. The second time you face the elite 4, after beating them, they have lvl 70-80. So, yeah, train your pokemon after you beat them.