You could go and try to recruit most of the legendaries with a secret slab or mystery part but some are hard to recruit so use a golden mask that will improve your chances you can get these items by using wonder mail
hope this helps
Then you have to beat Darkrai.
You have to beat the game to battle darkrai.
defeat darkrai mission
dont no
Someone may edit this but you can finally beat darkrai after you beat the ENTIRE game, not JUST the story mode.
You can get cyndaquil at Dark Crater after you beat darkrai's team.
go to darkrai level and beat darkrai
after you beat darkria and beat the final maze do whatever you want in the game.
You have Pokemon know the move discharge then take out the monster house Pokemon cause they are very weak! Then just go ALL OUT on Darkrai.
After you beat Darkrai it will come back!
evolve your main characters and try to "join 'em all"
You can't actually beat the game, but the farthest you can go is defeating Darkrai. You can be considered beating it if you beat him.