Mixed bulk sumia

She has low attack, so I think that if you are keeping close def and going with the whole enemy phase setup, then it will be better to keep reprisal lance for some actual damage.

Depends how aggressive you want to be and the buffs she's getting. Berkut's lance+def is the super-solid, slow (not as in her spd, but the pace of the battle) and passive play. Probably reaches, like, 40/40 or higher without buffs? And you put guard and she tanks all day and blows someone up with an ignis/glacies every other turn, or you put aether to be even more passive and she keeps her guard forever. But her teammates will have to make up in damage.

Reprisal lance is more double-edged. She'll take more damage, but she'll also repay a lot harder.

I would say shigure for mixed phase