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Both final missions seemed extremely easy, even borderline cheesy. In one of them, you can easily earn a platinum rating right away. However, I played on easy mode for a first playthrough, but without using auto-battles.
I ran through these final missions and leveled up two swords – Feather and Arachne – to level 4 and beyond. I guess I need to stop playing on easy mode, as everything felt too easy. The only mission that I haven't been able to complete yet in all the campaigns was the second mission for the Necromancer faction. I lacked the skill to preserve and grow my army.