Generally Assist skills buff the whole group. In case of rosenkreuz/solomon, they only buff themselves.
The main damage ability of solomon is solomonring. It does only 0.80-0.85x dmg each hit. This small multiplier seems to be weak at early-mid game, but benefits strongly from ability up in the late-game, once people have significant boost from attk up and element up.
Damage increase on paper
solomonring: 8% / 0.80 = 10%
an ability with damage multiplier 5x, 8% / 5 = 1.6%
For comparison, what does 20% attk up buff means, if +100% attk up and +80% element up are reached? (max. possible 160% and 200%+ atm)
weapon x element x ability x hits
(1+100%) x (1+80%) x 0.80 x 6 = 17.28 ( solomon ring no other buff)
(1+100%) x (1+80%) x 5 x 1 = 18 (shiva's single nuke no other buff, higher than solomonring )
20% attk up buff:
(1+100%+20%) x (1+80%) x 0.80 x 6 = 19.008 ---> about 10% real increase (solomon ring)
(1+100%+20%) x (1+80%) x 5 = 19.8 ---> about 10% real increase (shiva's nuke)
8% ability up
(1+100%) x (1+80%) x (0.80+8%) x 6 = 19.008 ---> about 10% real increase too, wonder?!
Solomonring gains 10% real damage increase with only 8% ability up, and that 8% is equal to 20% attk up buff. (big buff of Arthur for instance)
but what about a standard 1hit ability?
(1+100%) x (1+80%) x (5+8%) = 18.288 ---> only 1.6% real increase to non-buff state...
A strong single hit ability benefits less from that 8% ability up. 5x damage is almost best for this kind of ability, but it gets topped by solomonring with only 8% ability up. 18.288 < 19.008.
The math doesn't consider type advantages. Solomonring has all elements, only 1 of 6 hits can hit weakness. in case of type advantage, single element is still better.
The math only shows, how good ability up is scaled with abilities such as solomonring in endgame.
This game is balanced in Endgame content. The devs know, what they do.
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and I think you and felix are smart enough to understand the simple math, hm, felix, maybe not...
If you think anything is illogical, I'm all ears.