GW issues

I was in a similar situation than yours a while ago. Most of my toons were picked from here and there (Phasma lead, Kylo, Bobba, Rey and Maul) and they were nice and powerful, but not enough to complete the series of combats of the GW untill the end every time. Luck in the matchmaking was the key, and that lead to frustration.
Everything changed when i got the complete squad of Rogue One (Jyn lead, Cassian, K2SO, Chirrut and Malbus), then I learned that sinergies are the key, not luck anymore. I’m in love with this team and makes me smile every time I play it.
On top of that, now I have backups for the difficult fights or specific ones (empire and sith against enemies mostly jedis, First order, etc.). They are poweful, but my Rogue One team has the endurance I needed, and the rest of the teams provide them support.

When I find a Nightsister team like the one you show (or even worse) i send the FO squad on a suicide mission, with the only goal of reducing the enemy protection or even kill some of the rivals. Then my main team finishes the job leaving the fight with no losses 9 times out of 10. If something goes wrong during the fight, i quit and repeat until it works.

I read a while ago that the difficulty of the GW is determined by the power of the best 5 characters on your inventory. Make sure you have some spare teams before you keep pushing your main one.