Easy rank gaining!
When you find a mission wanting you to find an item make sure you have the requested item. Then,if you have that item, take it with you, take an escape orb,accept the mission and go to the mission's dungeon. When you start, use the escape orb and the pokemon which asked you to do that mission will take the requested item and give you the reward and some rescue points (if you use this trick on hard dungeons, you can take tons of rescue points!) (works on mystery dungeon blue and red, too). If you liked this trick please rate! :)