@Michaela: It's really easy! Just think of it this way. Your pokemon gets 3 levels for each of Cynthia's pokemon it fought, and 2 levels for each of any other pokemon it fought. If it faitns, it gains an extra level.
...So yeah, double battles earn you insane levels.
(6 and 6 vs 6 and 6 could theoretically earn you up to 25 levels on a single pokemon without the champion bonus. @_@)