Kerria needs heavy offensive support to unleash her full potential. But I guess she works very well with Saffron in a max offense team. Saffron with her Evade+Counter+Defense Stuff and Provocation and Kerria with her own Evade + team support full of Crit/heavy atk boost/skill activation girls.
I guess it can be slightly broken but you may loose some precious power/crit/skill activation if you use only 3 girl to support her instead of 4. Or you should use a girl with the skill act *1.36 based on skill level.

Spring Starflower or Pygmy Water Lily may be a good option after 6*promotion but you need at least two skill act *1.36 girls to finish your team.

Maple can be a good choice. +50% ATk for the team, skill act *1.36 and a lot of bonus damage for herself. Snowdrop and Easter Sneezeweed can be good too with *1.2 skill and +30% (up to 60%) ATK at the end of the turn. Since they supposed to last a long time, it's a really good ability.

There is also St Paulia post 6* promotion. Crit+skill activation.

There is so many possitilities. You must want to use skill as many times as you can but you also want to deals as much damages as possible. And you should want to use at least 3 Slice girl in this team to increase damage with personnal and whaleship equipement.