To actually get into the tomb:
Equip one of your members with long ranged weapons, keep your other party members far away out of the action. Now send your person with the long ranged weapons (with solo mode enabled) in to kill a few troopers, once his health is low bring him back to the other members and heal him with medpacs of jedi heal or something. Keep repeating this
Now the 3 sith masters:
This fight is actually quite tricky, although somehow (maybe luckily) I found it quite easy. The important thing to do is to save up your levels so that you can level up during the fight and get your health back. I used Handmaiden, Visas and Mira for this fight and only Visas was a jedi but still did ok. USE energy shields, they absorb a lot of the damage and use a jedi character to 'boost' your team with things like force speed and energy resistance.
Or..... hide behind a pillar and just take random shots at em or throw things at em
That works...but the easiest is to use force stasis (or insanity for darksiders) before the fight begins. It is also most effective to have 1 jedi in your party, 1 melee user, and a ranged user. Having the versatility to attack from all angles gives you enough firepower to beat all 3 of them. Make sure you have enough medpacks, stims, and other utlities too!