My lvl 43 Vanguard kept getting beasted by the rancor if I tried the traditional fighting style. So, I took an old strategy of just keep running from KOTOR with Dorne and it surprisingly worked on the first try. Do some initial damage from far away, stun it when it starts to come close, in this time set yourself up so you can go backwards down the long hallway and just keep doing damage to it with the instants. Eventually use your heroic to reopen the stun to get in more damage but as long as you keep moving and wisely time your heals (use Dorne) you'll eventually get its health down without taking too much damage.