Sometimes, when I get to the Boss platform, the launcher is sparkling. When I go ahead, it warps me to this arena where I battle a seemingly random legendary. I have befriended a Zekrom with a 2,442 power rating,a Heatran with a 2,265 power rating, and a Picky Virizion with a 1,934 power rating through this method. I also found a Giratina, but I didn't beat it yet.
I'm pretty sure if I got Zekrom, then Reshiram will eventually show up, too. Location, anyone?