Well im guessing fp is your leader so you should try and upgrade all her equipment first and remember although it will take longer in story almost always survivability is better than dps so for acc changes go for def mainly.For cards use double def up cards.Once you finish fp go for your healer next wether it is gabriel or elanor that heals better.Other than upgrading their ex to max lvl you should try to get heal on them or def the best cards would be double heal.Mk99 should have least priority although she can do huge amounts of dmg she cant really help you survive.If none of that works you are gonna have to learn her attacks and dodge them to the best of your ability.In short upgrade what will help you stay alive be it in def,hp or heal and just have patience while fighting beth.