A rank valks are better, most of my b ranks im not using them much except maybe for mei for the double twirl even though i got the beginning B ranks kiana, bronya and mei as SSS.

1. Get an active mentor and do the disciple quest daily to get free fire sakura.
2. Advise focus on free augmented (not awakening) Valkyria.

In typical battle she seems pretty not useful. However she might excel in some Memorial Arena Boss.
e.g. In Heimdall, having she equipped with Blood dance, she could just ultimate and instant break Heimdall 2x barrier and activate blood dance inflict damage amplification and tag-in burst character instant melt the boss down.