Make sure you have plenty of items and revive seeds because they are very helpful battling bosses and make sure your Pokemon holds a revive seed before the battle
it is impossible
after you defeat groudon
knowwhere
Defeat them while you're in their dungeon.
Defeat a lot of Pokemon.
you have to battle with lv 25 Pokemon
you find lots of shelgons after you defeat artucuno.
at final island
buy friend areas if ur lucky when u defeat a pokemon it MIGHT ask 2 join ur team
Darkrai is not available in Pokemon Mystery Dungeon: Blue Rescue Team, because Darkrai is a Generation IV Pokemon, and Blue Rescue Team only has Pokemon from Generations I-III (except for Bonsly, Weavile, Mime Jr., and Lucario, who only appear in the game as statues).
You can rescue Alakazam's team in Magma Cavern where you defeat Groudon That is the only place I have rescued them so far because i havent beat the game yet
All you have to do is defeat them and if you have their friend area they might join your team. (No promises!)