So here's the scenario. I'm the GM and I've set it up so the group (6 PCs) need to go to Dathomir to find a holocron. I am going to use the legends version of the Nightsisters and the players are aware of the force witches using rancors as mounts. I want the players to run across an old abandoned imperial outpost where they happen across a derelict ATST or ATPT, thought I think the players would get much more of a kick from the more recognizable ATST. They will need to perform some repairs on the walker but eventually they will run across a bull rancor and will need to fight. I think this will provide a very cool action beat to the story and bring in a lot of elements the players are familiar with in a an interesting way. I know the ATST is very powerful and could maybe one shot a rancor. However this isn't an ATST out of the box, it's been rotting in a jungle for quite some time. I will also say I'm not a rules lawyer and favor narrative over mechanics, especially for a memorable moment. How could I get this working without breaking the game?

Depends on how you design the Rancor.

First... Is it a Vehicle Scale beast? I like the idea that it's really a vehicle. Basically make it a SIL4 beast, with nasty claws and vehicle scale stats. On human scale its Claws might be Breach 1, 20 dam+successes or something hideous like that. Give it stats as a light vehicle, its claws being vehicle scale weapon and let it pry it's way into the AT-ST then it's a whole lot more scary for the crew, especially once they know one swipe from it will be fatal. If it is SIL 4 it is at a penalty to attack human targets, so it might aim more for the AT-ST to draw it's attention while those on foot maneuver to throw thermal detinators at its feet.

Give it some kind of special maneuvers or attacks it can use that the AT-ST can't. What if it can knock an opponent prone? What if the AT-ST is knocked prone? How hard is it to get up?

Give it some kind of advantage in some way to make up for it's lack of ranged attack possibilities? Can it throw rocks? Can it throw mud to blind it's foes?

Penty to consider.

How about a Rancor and ATST! Rawwwwww!!!!!

Since my players are on Dathomir too, I designed a similar situation

First of I used the "captured Rancor" stats from the Eote book.

Added +5 duarability (didn´t want a wannabe jedi to lucky onhit crit it)

Advasary: Groupmembers +1 (since you have 6 PCs may be stop at advasary 5)

Wounds- and strainlimit: Basic+ 5 for every groupmember

Silouhette: 4

"Weapons" as standard + Metalbraking claws (Spend 3 advantages to reduce the armor of a hitted vehicle by 1 permamently) [so the AT-ST has an deadline for use]

This nemesis may use an extra action at the end of each round.

And one more "homebrewed" Talent: Bloodthursty Eye: suffer 2 Strain to downgrade the difficulty of an attack vs an target that was wounded within the last round.

also rember that the attackrange of the Rancor is short (NOT engaged)

Since the ATST is patchworked the weapon was "only" considered as a heavy repetierblaster and a rocketlauncher (one shot left) and used personalscale ranges

I build that encounter so that the rancor would start to use The Bloodthursty Eye talent starting when his Woundthreshold was cut down by half or then the critical wounds had stckup to three times. Fliped a DS destinypoint and calling the players that the Monster in front of them gets really angry and enrages.

Happy hunting!

Adversary 5? We could make that rancor fight vader and still have a chance to win.

Rawwwww! Rancor Smash puny walker!!!! Rawwwww

yup that´s what I had in Mind.

Not every rancor is a cute little puppy like Jabbas.

Kinda looks like a Rancor..........and it's awesome.........

Rawwwww! Rancor Smash puny walker!!!! Rawwwww

Came here to post that