Whenever I have to fight a Sith (Elites) even 2 levels lower than me (at 4 level lower, I can defeat them with few HP to spare), I tend to die. I play a 32 Trooper Vanguard and have level appropriate armor, weaponry (minus a couple of mods/enhancements that I need to level) and all class/vanguard abilities trained. I have Elara as my companion set to her aggressive healing setting.
Very occasionally, I'll have problems with elites having large damage rapid fire weapons, but slightly different tactics usually leads to my success. This does not happen with attacking Sith Elites
EDIT The one I was specifically referring to this time is the Sith Apprentices (Level 28) on Alderaan. These are one at a time.
My stats
Skill tree
Is this a simple need for better armor/shield/enhancements with more Damage Reduction, Defense Chance, Shield Chance and Shield Absorption? I've been concentrating on Aim, but perhaps I should adjust my priorities.