I've always said that the only thing good about the xenophobes in Goldenleaf is their choice of Pokemon type (Ghost being my second favourite)
Now, down to business.
Some more things to know about the Haunted Field:
Dark and Fairy type moves are nerfed. This shouldn't be too much of a problem if you've got a bulky or speedy offensive Ghost type (probably why Jan didn't add in the Gengar line).
The field can be switched back from the Blessed Field easily by Narcissa's signature move, Ghostly Wail. This, coupled with Xatu's weakness to Ghost, is why I wouldn't recommend a field manipulation strategy here.
Moves like Flame Charge, Flame Burst, Fire Spin and Inferno deal additional Ghost type damage. It may be to your advantage to pack these moves.
Also, for your own sanity, before you get to the point where you have to fight Angie, make sure you have multiple, viable Pokemon that can defeat Ice types.
My last time, my only answers to Ice types were an Escavalier(which outspeeds almost nothing, btw) and a Magmar with a Jolly nature. Finally, I decided to hightail it to Carotos mountain, obtain the Cyndaquil egg, train it up and use Typhlosion's Eruption to even out the odds slightly.