After it was send to the (most likely) past and attacked in middle of the progress by
cresselia, he got amnesia and then sometimes appear in dungeons, you'll need a mystery
part or secret slab in inventory to find him and a amber tear or golden mask in hand to
increase the chance to recruit him, you'll also need strong pokemon to take him down,
he'll appear in miracle sea 3f, lower crevice cave 2f, mt. Travail 17f, spacial rift 14f,
mystifying forest 13f.
I still don't know how to get team raider off the beach, but it might happen when at some
rank suchas the guildmaster rank.
You can't finish the whole game, you'll keep playing it until you make a new game or until
you stop playing it, pmd is one of those games in which you'll keep playing without an
end, such as other pokemon games, harvest mooon, animal crossing, fossil fighters, and
mario games.

Thanks for the answer.i will probably recruit darkrai tomorrow of i can since i got the
mystery part,secret slab, and amber tear