GW is an ever changing challenge, everyday is different. I love it. I started playing a couple of days before New Year, now I started beating GW 5/7 days and all was right in my world. Then all of a sudden I couldn't beat it. I was only winning 1/7 so I beefed up my supporting teams and started to be a bit more tactical. Back up to 5/7. Then I started regularly coming up against Chaze and Thrawn and etc and started to struggle. So I started preloading squads for the first 5 battles. Up to 6/7. Now G12 has come along and this time I'm ahead of it, because I just dropped a Zeta on Finn.
My point is GW is an adaptive part of the game. If you keep doing the same stuff, you can expect to get progressively worse results...