I found a possible solution.
After the the two Jedi engage in battle, drag the Sith well away from the fray via aggro. After that, you can whittle down his health like any NPC.
I did this by disappearing (a possible smuggler/IA heals only talent), then resurrecting Corso and engaging the Sith on the Bridge, well away from the two Jedi ladies fighting. This was as a heals smuggler and after realizing I could sustain my toon while watching the fight more or less forever. It's not clear if non-healers can force the Sith to leave the battle perimeter, but seems possible.
Edit: finished quest as level 26 smuggler/sawbones