You could always scout. Put it up and request a legendary or something of the sorts. If after a day nobody traded with you, take it back and ask for something less valuable (trade evolutions, etc.).
Repeat that process until someone trades with you and then put another Phione on there and try again. At some point you get a feeling what people will give you and what not.
I've completed the Pokedex of a couple games that way, so I know that that method works. :)