Oof. I didn’t realize how young your account is. Based on your toons, try to avoid the NS until you’ve farmed sabine. :D
At this point, if you must face the NS, I would stick with Kanan and Chopper and dump Zeb for thrawn. Double down on the healing and use Thrawn and Kanan’s protection recovery instead of Zeb. Zeb isn’t much of a thug until he’s higher gear and has full abilities.
You’re on a younger shard. If there’s lots of players in your shard using Nightsisters, swap it up and farm Phoenix on the sly while you focus your farm on an Empire squad for a bit (4*or 5* them). Lots of arenas are rotating between teams now due to the diversity in competitive arena teams.
You could have a pretty good NS killing squad there, if you used a Thrawn/Tarkin lead and worked on them. Stormtrooper is a much better choice and much easier farm (cantina store) than Royal Guard. Snow and Magma both have aoe so use either one of them.
Thrawn, Tarkin, Tfp, Storm, magma/rg.