Phasma, sadly, has been rendered largely obsolete since the release of Matt the Radar Technician. And unlike Finn, she doesn't have enough leftover First Order toons to field a B team, though filler toons could take advantage of her lead.
KRU is best lead and best tank.
FOST helps because First Order does well under double tank, and he's just an overall very good unit. Counter chance with speed down and a dispelling basic with advantage? Yes, please. His zeta is great, ally TM gain and self cleanse on his taunt ar great, and a 3 turn cooldown on his taunt is great.
First Order Officer is the most important First Order toon. Cool down reduction, TM drain, huge TM gain, spreading advantage and offence up, cleanse, this guy brings a ton of support to the table.
Some combination of Kylo classic, FOX, and FOTP. Straight up kill. They all have their ups and downs. FOTP's zeta puts him on par with the other two main beatsticks, and that zeta adds a huge amount of much-needed survivability with all the foresight they'll gain.