Because I have all level 100 Pokemon and want to battle someone strong, and I'm bored.
You can battle the elite four as many times as you want, and to top it off, after the first time you defeat them ( You know, with N and Ghetsis ) All of the levels go up .
what level were your pokemon to rematch the elite four. most of mine are only in there 60's
Or try big court or small stadium