Damage issue

Only like 3 of my expeditioners are able to actually deal damage (Verso, Monoco and Maelle) despite me getting the supposed recommended "best weapons and best builds" for the characters they still struggle to deal over 100k. The boss in question is >!Simon!<, where Verso deals about 1.1m with Berserk Slash on S at 1hp, Monoco around 800k with Création Void and Maelle 800k-1.2m with Stendhal, while Lune ends up dealing only about 100k with Elemental Genesis and Sciel around 70k with EVERYTHING. Someone please help.