Pulled Diago and died many times. I found that concentrating on the Sith first was a good solution. Diago is a hard fight, the Jedi will not hold threat. The only thing that will save you is interrupts. Every time he goes for Med Scan, hit him with Flash Grenade or Distract power. Even then, settle in for a ten minute burn-fest. Corso or Bowdarr, will not survive, and you only barely will. Hang in there and plan on either grouping or redoing this multiple times.