lol, I don't know why people are saying you need better gear. Level 45 quest reward with an ilvl45 torso is enough.

Have Titan- egi tank Belias and burn down the Tristan. Belias will come over to you to use a one shot aoe, make sure to dodge it.
Once you dps the boss down enough, he will dismiss Belias. You can switch to Garuda or Ifrit at this point. Kill the adds as they spawn and dodge Tristan's AOEs. Keep burning him down and you win