Is Raikou necessary also in Solomon/ Demon Pillar fight?

As I remember, Demon Pillar’s normal attack will hit single target except the crit. So if you protect her probably and finish it fast, it’ll be okay.

She isn't necessary. Her crit potential and third skill would give her some decent damage against any of the demon pillars, but you are right she's not the best choice since she doesn't have the defences to keep alive for long. If you want to use her for the damage and have the set-up for her she is still usable, but I personally wouldn't.