All I need is get past P1, Kerria should be able handle the rest.
Sorry but I don't think. Kerria can take 4, maybe 5 hits (with guts I mean) from the last form. Evasion is useless with boss' accuracy buff and it hits too hard.
I made a boss battle video where Kerria and her friends fight the last form: https://www.youtube.com/watch?v=v-o5exLIKiQ
I thought they will not finish it. This boss is too crazy.