It's true Kerria is a fantastic choice, but lets not forget the hard work of the other girls in her squad that gives her the opportunity to excel
Between the choice of Kerria and Alpinia, I tend to side with Alpinia for the counter and speed boost's utility outside of high powered raid boss events, but that is just my personal preference. When I pair the two together though, the damage to those bosses jumps from four million for one squad with just Kerria to eight million when I use both. Alpinia might be the inferior choice by herself, but definitely matches Kerria pound for pound on the damage when they are side by side. Considering that Kerria needs the damage reducing girls to do her thing as well, I say they fight bosses pretty equally overall.