Welcome to Smogon! Take a moment to read the Introduction to Smogon for a run-down on everything Smogon, and make sure you take some time to read the global rules.
Koraidan is 4x weak to fairy I use tera fairy gholdingo to counter it and player can use flutter Mane with booster energy you can out speed him and ko koraidon but it's isn't same for miraidon