I’ve said many times that IG90 lead has given me consistently the best results. There’s an argument for STL lead when you first start a new tier, as he offers more survivability, but IG90’s payout gets easier and easier to achieve with some of the newer units in the lineup.
For the single target bosses, I’ve always found IG90, STL, Choda, Assajj and Barriss to be the best lineup (until I got Maul). The technique is largely the same - blast the damage asap, use Assajj’s buff on STL (to increase his speed) and try to time your use of Choda’s middle move to give foresight before the enemy uses an AoE. Barriss’s constant assists help so much against these bosses.
For the multi target bosses, strategy is much more nuanced. For the guardians, I did use STL but timed the use of his mass assist move until I was certain I’d defeat a guard outright. I found Barriss problematic against the guardians, as she would frequently assist and remove shield stacks when I didn’t want her to, but I tried to plan my attacks to minimise the negative contribution.
For the cats, I didn’t use STL’s second special at all until both were at very low health. There’s an icon appears when they have too big a health difference - pay close attention to that and make sure you hit the higher health cat as soon as that icon appears.