3 main ways to improve:

1. Make your whole team mono-element, in this case water. Find some water staff or something for Cass if you want to use her as soul. Also replace Baphomet with Oceanus. Amon is okay with the skill to block affliction but she isn't taking advantage of elemental bonus.
2. Make your whole weapon grid mono-element. Yes SSR weapons do seem attractive by stat but those +HP and +atk bonus from the wrong elements are not working for you. I.e. Susanoo's thunder sword isn't giving you as much bang for the buck.
3. Your eidolons need boost. Feed them a lot of upgrade material.

Also: analyze why Expert didn't go well for you. Did you lose too much HP in the pre-boss mob fights? Do you have insufficient damage to kill boss before your HP drains away quickly? Did the boss cast any buff/debuff that mess up your strategy? Isolate the cause and work from there.