From my coop play, the best defense is being a hero character yourself and popping a special move. That and focus fire from your allies. Short of being a force user yourself that’s about it. Chewbacca with his middle ability does some good damage if you get headshots. Especially when the ability is upgraded.
I’ve also had success as the enforcer super droid when playing first order. No heat generation + increased rate of fire = delicious. I can keep up with hero characters with that dude.