What i say is go Chansey hunting then go to Backlots mansion.
You need high levels and you will want to be higher to stay safe.
Yeh its time consuming but its worth it in both the long and short run.
The lucky egg is incredibly usefull and will help not just for the league but after as well.
But when i did it the league continuaslly and very quikly my levels would grow untill i had one pokemon that could take out almost every person single handly.(on my perl this is, on platinum i did the league first time)