I don't get it :/

My party seem to be suffering from a mid-life crisis, I'm trying to get passed this one player in Chaotic Expedition, They seem to be less than 10% of my BR, yet I'm losing to them each fight, no matter what I change in my party. Can someone explain this to me please, in little and easy to understand words (that's a joke, this is not). FYI me party consists of Blood Angel, Rajah Skyscream, Radiance Knight, Shadow Kitsune and Nina. All with their own amulets at full strength etc.