I suspect you're either on Voss or completing the Hoth bonus series.
In anycase... towards the end of the game, most MOBS mainly do unresistable damage (internal/elemental). You take 100% damage. Even with 10K hitpoint, they can burn/poison/bleed/shock/etc. you to death very, very quickly.
You kinda get an idea of this as early as Quesh, or Tatooine - Czerka lab infected mobs.
I don't think you're doing anything wrong. Not a lot you can do, except the usual and just have the brute force it.