There's a thread about this somewhere, I believe under Game Discussion, but typically, people use Quest 24-6.
Use all your allies, and only 1 of your characters, and he/she will get 500+ xp every 5 mins.
Also people tend to feed level 4-5 2* cards into the card they are trying to level.
That's it in brief, but there is more detail elsewhere.
Cheers, and Good luck!
I once punched a man @ level 1 and when he came to he was level 9. He was quite thankful to me, needless to say.