Niantic follows up the tease with some official details
We recently posted about the impending arrival of Trainer Battles in Pokémon GO, which Niantic listed as coming "soon" at the time. That time frame has since been narrowed to sometime this month, so get ready to use all those Pokémon you've been hoarding to take on friends and enemies alike.
Niantic's latest update provides a helpful outline as to how the whole system works. All you need to do is scan your Battle Code with another device, assemble your team of three Pokémon, and battle for a chance to earn rare Evolution items and other rewards.
As previously outlined, you'll need to be in the same Trainer Battle League, which dictates the CP limit for your 'Mon. Great League's limit is 1,500 CP per Pokémon, while Ultra League's is set at 2,500 and Master League has no limit.
Battles are handled in real time, so you'll want to prepare with Stardust and Candy before unleashing Fast Attack and Charged Attack moves. If you time it right, you'll be able to deploy a Protect Shield to prevent critical damage, but there are only so many chances.
Check out the shield, battle field, and menus:
To battle your average Trainer you'll need to be nearby, but Ultra Friends and Best Friends can challenge one another from any distance. You can also get some practice in by battling Team leaders Spark, Candela, and Blanche, with rewards being divvied out once per day.