Uhm... Beat Susamaru in bad route shouldn't be that hard. But just in case, use only the tentacle skill and defend when she use her ultimate.
And about the side quest in good route. As it is optional, I thought about making it more difficult, but it is not impossible. Just manage your energy well, defend yourself when she uses her ultimate, use potions, bring an assistant which attribute is the enemy's weakness. In this case it is water (kanao). Still, I know the battle system needs a lot of improvement.