you may have 2 evolvers to beat her
Like Monferno and Luxio
Or Prinplup and Staravia
-I just beat her with D/P prinplup and staravia-
Or Grotle and Bibarel
Or you may have lv.100 fire pokemon
from tradelike:
Infernape,Rapidash,Or something Like Typsolhon here is it
You can also trade arceus while its type is FIRE
or Heatran
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After you beat her with your 6 pokemon
She gives you togepi egg.