I find the frenzy damage reduction as a berserker makes Karlach very tanky combined with her Tiefling fire resistance as well has her getting 1 cheat death per short rest. Gain immunity to charmed and frighten from mindless rage. Reckless attack helps immensely early game when my hit rate was only 50-70 on top of the barbarian also getting more attacks then fighter from level 3 until 11. Feral instinct makes it so you can't be surprised and gets extra initiative. Karlach would often move first and always before my casters letting her easily throw water to let my cold/lightning spells wreak havoc. She could then easily get into position because barbarians also get 5 extra movement speed. Due to me using her as a priority target killer she rarely killed stuff meaning if I used fighter the same way great weapon master would rarely proc for that 4th attack meaning both barb and fighter would be consistently doing only 3 hits. Sure the fighters superiority die will make them do more damage than the barbarian but I can easily make that up with the initiative and movement bonus letting me get into position and setup combo's. Might also be some kind of crit build to leverage brutal critical to make up that difference.
