I agree with @Telaan
To explain it a bit more:
Phasma can hold her own with 4* for a while. She is not fragile, and her main strength are not star-dependent. She will give you %assist chance, AOE slow, and Turn manipulation regardless of stars
Lumi you want her to heal (dependant on her HP, which means dependant on stars) and to do damage (dependant on stars).
I don't mean to sound mean as I have been there as well but your team is still so young that you shouldn't worry about arena unless it is a new server you are on and getting top 50 because of that. Lumi should be leading your galactic war team so focus everything into her.
☮ Consular ☮ Many of the truths we cling to depend greatly on our own point of view. -Ben Kenobi
Lumi.for gw definitely BUT....if you're focused on arena rank...I'd go phasma. I just switched lumi for daka and the difference is night and day. You only need one heal if that for arenas. I jumped from being in the 200-100 bracket to breaking top 50 on my server. I'm 100% ftp and if you're serious about keeping chewie he can self heal as is. And your team is lacking serious damage so go phasma imo.
Replies
To explain it a bit more:
Phasma can hold her own with 4* for a while. She is not fragile, and her main strength are not star-dependent. She will give you %assist chance, AOE slow, and Turn manipulation regardless of stars
Lumi you want her to heal (dependant on her HP, which means dependant on stars) and to do damage (dependant on stars).
My team is;
6 star sidious
5 chewy
5 JC
4 Fives
4 potentially phasma/luminara
In terms of damage who would you recommend I farm? I like chewy though because of his self healing, taunt and quite high hp.