Elemental (dis)advantage is added to Elemental Atk. The multipliers to base Atk are, off the top of my head, Assault, Elemental, Crit, Vigor, Union Guardian, Special (which we'll have starting next week). Those are the common ones, then you also have Berserk and Stun punisher (which don't stack as they're in the same modifier for some reason and overwrite each other despite the weaker still showing up?!)... was that all?
EDIT: Har, while typing this in the past five minutes there's been like three posts.Solomon sure, but... are you really going to swap Souls once you have a Herc/Shingen Soul weapon? I don't think so. You pick one and you level their weapon and you stick to it for a long, long, LONG time. If not forever. You can upgrade from Herc to Shingen if your team improves, but outside of that...