That was a wrong descripton from me in my first post.
This way to correct:
An ablilty which hits 6 times or more, usually, has low damage multiplier, around 1. (1+0.2)x6 =7.2, 20% ability up means true 20% up.
An ability which hits 1 time for 5x dmg is different. It will deal 5.2x dmg, (5+0.2)x1=5.2, 20% up means only 4% true increase for this ability. (Because 0.2 / 5 = 0.04)
The ability up is added to the ability multiplier.
The attack up is added to the character attk up multiplier.
Character attk multiplier x element attack multiplier x ability multiplier = total damage multiplier.
It really depends on the playstyle and kamis in your team, whether solomon is good or not. Sussanoo's first skill for example will benifit a lot from solomon with Rosenkreuz's EX.
Without certain kamis in team and low weapon assault skill level, solomon is pretty useless. So poeple should focus on other souls first, but if someone says "Solomon is absolute crap", he needs to learn more about the game.