The assist skill should provide buff for the active team. There is 1 dark SR, who provides even buff for whole in the sub slot.
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I also say, solomon is useless, if certain requirements/SSRs are missing. And you don't need to consider her below rank 70. I never said, she has high priority on the unlock-list, because mordred/d'arc/andromeda are more beginnger-friendly.
Soul points won't be a issue, when you play long enough.
If you try to level and use solomon instead of mordred/andro etc, then you do something wrong. The first word I used for her "Endgame soul"
Don't miss the point.