Swoobat is pretty cool for a Psychic-type. A Psychic/Flying-type, at that. If it has Simple all it has to do is set up Calm Mind and go to town.
Otherwise, Gardevoir has already been said and is very useful here. If you don't mind grinding up another Fairy-type, most of them will do pretty well- Swirlix and Spritzee are available, and you can also get Igglybuff and Cleffa, though I don't know how you'd feel about grinding those up...
...And I haven't tried Meowstic against her, but I would love to. I don't think it'd have Reflect by that point, so it can't buffer physical attacks, but... hmm...
Oh yes, and Ghost-types are pretty useful too, if nothing else. You can easily catch Duskull and Yamask in Shade's Gym, for a couple of them. Both very bulky and capable of taking hits, but rather slow, unfortunately.
Edit:
X items are broken as of Episode 11, so that's probably not the best idea, but Bulk Up/Dragon Dance could work just fine, too.