I have zetas for Phasma, FOST, and Kylo. I'm having a pretty rough time with them. Most of the guys in the FO thread say they fall into the 30's-50's at night and do 5-10 battles to fight back up each day. So far I've just been using my extra battles to get some practice in before switching back to my Rex team. Depending on what you are going to have to fight everyday opponent-wise FO could be good or bad......
https://www.youtube.com/watch?v=uUDejuoqwOE
https://www.youtube.com/watch?v=cKix66fJut0
https://www.youtube.com/watch?v=xDPHpBfQsu0