"Champion's" Pokemon

(Please note this is just speculation. It's easy for me to suggest this, but I haven't gotten to this point yet either, so the best I can do is go by types)

Spiritomb 61 - Ack! A Pokemon essentially without weakness! Ghost Type covers Dark's weaknesses and Vice versa. There are two ways of handling this one.
1.) Purely overpower it, or
2.) A funny one. The "Identify" moves apparently nullify immunity, which means they should allow a Fighting type to connect (Which Dark is weak to)
Gastrodon 60 - A very fast Grass type should do the trick...or one that can survive going last. Only because he's water AND ground type, which makes Grass moves do 4x Damage.
Milotic 63 - This one is never fun...Water dictates that Grass or Electricity will work, but these things are always full of tricks.
Garchomp 66 - Another fun one...if you have a strong Ice Pokemon, this is the time to use it, or beef one up. Flying may be an option, as it negates Ground type moves, but...(Kinda makes one wish Articuno was available by this point)
Lucario 63 - Your best bet is Flying as the Steel negates it's weakness to Psychic (Not to say it doesn't work...just not super effective)
Roserade 60 - One fast, powerful Psychic. Or, provided Bronzor learns Psychic, and is powerful enough, it could be worthwhile, as Steel is either resistant or immune to both his types.

This is pure speculation, so I take no liability if you still get creamed. Just remember...unless I'm mistaken, you keep your levels if you game over, so just keep hacking at her, and eventually, you'll overpower her.

Good luck!