I played and beat Star Wars Jedi: Survivor before it officially launched, so believe me when I say I knew the woes of fighting the devilish Rancor on Grand Master difficulty with little-to-no help. I even did the fight before obtaining any special abilities or new stances (outside of three you technically start with).
You're not familar with the Rancor's moveset yet, but don't worry, you don't really need to be. There's a few things you need to do to ace this fight without getting hit.
My biggest mistake in this fight was playing it like it was Dark Souls. It's not. Typically, in a Soulsborne game, you want to jump in as close to the enemy as possible because it provides more opportunities for you to strike as they recover. DO NOT DO THIS HERE.
I love Star Wars Jedi: Survivor, but it's no where near precise as Dark Souls, so therefore we can't rely on the same consistency there. The rancor has swiping moves to instantly kill you on Grand Master. There are plenty of methods out there, but this is mine. Hopefully it works for you!
So let's jump right into it.
How to beat Rancor in Star Wars Jedi: Survivor
First things first, equip the Single-Blade lightsaber. You need the speed and power.
KEEP YOUR DISTANCE. Wait for the Rancor to attempt an unblockable grab attack (highlighted in red). You can bait the Rancor into doing this if you manipulate the distance you are from it. If you're too far, then the Rancor will either jump to you or try ranged attacks. We don't want that, so get a medium distance away from it relative to the battlefield.
When the Rancor commits to the attack, you can easily dodge it from the distance you are. Now jump in for a quick light combo. As soon as that combo is finished, you quickly dodge away.
After this, the Rancor will likely slam the ground for a melee or ranged attack. If you find yourself getting caught by this, JUMP. It took me a while to figure out you can't dodge everything and you need to jump over some of these attacks to ensure you're not caught in them.
You should be able to rinse and repeat this method until the Rancor is defeated. Do not get impatient. If you want to speed this up a bit without risking death, you can throw your lightsaber for some additional damage.
That's how you beat the Rancor with as little strategy as I could throw at you.
Good luck, Jedi!