You need to kill R2 first, then Nihilus. After that, you take down Barriss at your lesiure. This is the best way. Ideally you fracture GK if you can, but I have gotten away with using buff immunity on GK while I killed R2 and Nihilus. Again though, R2 should be your first target to get rid of the extra health/protection and that annoying stealth before you work through the rest of them. If you don't have Thrawn, I would probably try to keep Nihilus stunned (after R2 was dead) while I worked on Barriss.
I hated the GK/Zarriss combo until I figured this out. Now I beat them quickly.
