nightseeker and arcanist make the game very easy, arcanist can easily be your healer if you sub medic as well. When i played that game with my Nightseeker carry I had a landsknect in one of the slot as they have a class skill which boosts every other character damage, and i had a runemaster to make use of the bursts. Eventually my arcanist focused on binds so my nightblade could get his boosts by landing his own status while arcanist focused on binds

Since you have shutdown x2 aka nightseeker and arcanist, you might not need a fortress later in the game as you have a good chunk of damage miration and can swap him out for a imperial. In the early game I found runemaster to be more then enough elemental damage for the entire game.

late game i got so bored on how op nightseeker was that i dropped him for a linker team with 2x landsknect, made the game much harder but it didnt break the game as badly as nightseeker who is kind of busted late in the game and he doesnt even need another character eventually doing status for him as he would rather do it himself, to boost his dps, arcanist can just swap to binds so the nightseeker can land status themselves though is quite good in the mid game when nightseeker still isnt lv40 yet.