get him/her to send you their team and go into the dojo and you will be able to use a new cave. go into there and fight his/her team.
Fight him alone with friend bow
You fight him at Spacial Rift.
You fight Lugia at the silver trench 99f. In Pokemon mystery dungeon 2 you must bring mystery part to surronded sea 18f it will be walking around the floor.
go though a dungeon with cresslia and when you reach the end you will fight darkrai
You just fight and defeat them and then they will either join you or they wont if you find a friend scarf though you can increase the chance of them joining you
Fight Darkrai
You can fight him over and over again or use the friend bow, the friend bow is in special missions with codes online.
i know in Pokemon mystery dungeon blue you don't need to buy or get it you need to fight him and let him to join your team say yes and you got rayquaza and its friend area!
Uhh fight it? that's all you gotta do is level up and fight it...
There are 19 floors in his dungeon. Then there is the summit were you fight heatran ( so 20 overall.)
fight them good and then they might ask to join you!!
go thru STEAM CAVE and fight it if you kill it you can fight it again..veronica