Count yourself lucky! If that KRU had been really well modded you wouldn't have won at all. KRU with 3 health sets and 4 6dot health primaries is nearly impossible for CLS/Han/Chewi and C3P0 makes it even worse. The unique ability combined with leadership is why.
... When a First Order ally gains a status effect, they recover 5% Health and 5% Protection.
and
When Kylo takes damage, he recovers 8% Health... Kylo takes reduced damage from Percent Health damage effects...
In this context Chewi does 0.01% (instead of 20%) max health damage to health damage resistant characters like KRU. KRU then gains 8% health because he was damaged. If you add C3P0 you'll also apply expose on basic. When the expose is triggered by another attack it deals 0.4% damage to KRU (rather than the normal 20%) and then heals him by another 8% health and that's on top of the 5% health and protection he gets from every debuf you land on him.
It is admittedly quite difficult to get KRU to high enough health to survive a g12 rebel team. It's mod dependent of course but >65k health with a bit of extra defense too should manage it against most. And you might as well mod for health because even if extra tenacity instead would help against palp or JTR a super fast FOO or OG Kylo is more useful (and to be honest trying to defend against a good JTR at least is pretty much a lost cause).