Generation 5 is the easiest generation of games to train Pokemon, with the exception of Gen VI.
All you need to do is find rustling grass and battle Audino; they give more experience than most trainers do. You can get a further boost after Mistralton City, when Professor Juniper gives you a Lucky Egg at the Celestial Tower.
If you're facing Burgh, then there is no readily available grass around for you to train. Sail back to Virbank City and train against the Audino in the Virbank Complex for Burgh.