The 2nd easiest way is to either train.

the way i beat her is train,get souls, use them to get my second fire form(and other elemental forms), next i attacked the blue creature first(because i failed when i attacked the red one first),the blue one dies easier to fire crest magic,keep attacking only the blue one till it dies,next attack and kill the red one,make sure to be at full health and all,after u win you'll get fifty soul pionts.
godd luck,hope i helped.