vs. Shadow Triad
The Shadow Triad of Team Plasma is available to battle in Icirrus City. They use a variety of battle styles against you.
From the Pokemon Center, just head all the way to the east, across the bridge until you reach what appears to be a dead end with an item on it. The Shadow Triad will appear out of nowhere and challenge you to a fight!
The first member of the Shadow Triad is fought in a Single Battle, making the fight rather straightforward.
Afterwards, you'll have to fight the second member of the Shadow Triad, except this time in a Triple Battle. He'll ask if you're ready to fight him, and if you say no, the remaining two will disappear for the rest of the season you're in.
Being a Triple Battle, you'll want to make sure you have a sturdy Pokemon in the middle. A Fire-type is your best bet, but a solid Fighting- or Ground-type Pokemon should sit there comfortably.
After beating the second one, there's only one remaining member of the Shadow Triad for you to take on! This one will fight you in a Rotation Battle. Like the previous one, he'll ask if you wish to fight, and if you say no, he'll disappear for the rest of the season.
Fighting-type attacks may seem tempting to use against him, but since his Banette is completely immune to them, you should try to stick to Fire- and Ground-type attacks instead until Banette is out of the picture.
The Shadow Triad will reappear every season (month) for you to battle again, although their levels remain the same each time.