If memory serves, the turrets have an instant kill status or something on their utc files (I'm assuming it's utc, though I could be wrong). It's similar to encountering Calo Nord in Javyarr's Cantina, he has the instant kill status and invincibility. You'd have to go into the .MOD file of the Lower City and manually edit the utc files for the turrets and the Sith Soldier there to remove the instant kill and make him weak as anything. This can be done with the ERF tool to open up the mod file.