IMHO Herja is completely useless in banish as you need as many heroes attacking banish not focusing on your other heroes
For me, Calvin and Shayd are brilliant alongside with flavius who take away banish morale so he is forced to do one by one which is kind of the purpose of calvin
Overall, it depends how strong you are as I am only sitting at ~40 mill power (Yes still a baby

),
What would be nice is if you could put maria in the slot where you would usually put herja and for example if you fighting against a not so big banish like 250 mill when you are big enough to not have a banish attack completely wipe out one individual hero in one attack, then maria could constantly be healing them ultimately resulting in you destroying banish much quicker