In Nimbasa City you will find the Battle Testing Building. It is located in the house directly to the left of the Pokémon Center, and is only accessible after you have beaten the Elite Four. It is a facility which allows you to test your battling skills, as I'm sure you might have begun to suspect by now.

There are two different battle modes which you can practice here: Single Battle and Double Battle. You face 5 consecutive battles against different trainers, increasing in difficulty with each battle. Even if you lose a battle you will continue. Even if you lose all of the first 4 you will continue to the 5th and final battle, although, let's be honest, you may have thrown your new shiny 3DS against the wall by this stage. All Pokémon are standardized to Level 50 and Legendary Pokémon are a big no no.

After all 5 battles you will be given a certificate giving you a Battle Rating. You are rated out of 7 stars, and if you have a shred of common sense about you then you can probably do better than my pitiful 2 stars. While a detailed points calculator is currently unavailable, you are given more points for knocking out Opponent's Pokémon and keeping yours from fainting, as well as showing good battling strategies and type match ups. You lose points for having your Pokémon knocked out, losing battles and not having EV trained your Pokémon to within a millimetre of their 2-dimensional lives (probably).