tl;dr: add an extra restricted legendary to your series 11 team. aka GS Cup
RULES:
Singles: bring 6, pick 3. Doubles: bring 6, pick 4
Your team can have up to two of the following:
Mewtwo, Lugia, Ho-Oh, Kyogre, Groudon, Rayquaza, Dialga, Palkia, Giratina, Reshiram, Zekrom, Kyurem, Xerneas...