Galactic war has always been a difficult challenge. It used to not even be simmable, and then it used to take 150 victories to even be able to sim it.
Nightsisters are a very difficult team to beat unless you have one of the teams that counter them. If you get a team that is a "hard counter" to them, it becomes very easy to beat them. If you don't have a team that counters them well, your battles are going to continue to be very difficult, and very RNG-based.
Many other people have posted great team compositions that will destroy nightsisters, and I would recommend that you get those teams and gear them up if you want to be able to defeat nightsisters.
The 6th, 9th, and 12th nodes are designed to be difficult to beat, especially the 12th node. It isn't supposed to be easy, that's what all the other nodes are for.