Nexomon

You can catch Arqua and all the other Legendaries in the post game. Once you collect 41 nexomite fragments return them to Miriam for a radar.

Once you have the radar you simply need to travel the map waiting until the message that a legendary is nearby appears at the top of your screen.

Important note: Which legendary that spawns is completely random. Even ones you have already caught can still appear. Best bet is travel back and forth between two maps.