OOT Incidental to bind the AT-AT after the pilot announced he will do a pilot check to stomp the guy. A very liberal use of bind with mastery upgrade to apply a critical hit in a discipline with two setbacks (for the aiming on the movement systems and thus choosing the component critical hit) against impossible. But this would be really a very liberal use of bind, a special rule for Vader. Nothing wrong with that, adverseries are not supposed to follow the rules for PCs, so that stuff is cool.

Just make sure to turn the destiny point to give the players at least something when they are trying to escape from Vader next.

If you want to allow the players something similar, I would require the use of the magnitude upgrade once or twice per silhouette beyond 1.

Move power, specifically with a few Strength upgrades, a single range upgrade and all of the control tree taken.

Firstly, a GM could rule that you need the Magnitude upgrades, and that you would need to autofire the four limbs and head using the 'throw' ranged combat check at the target (the body), and use that (silhouette x 10) damage to reflect what happens on the scene. I imagine the giant legs and head are silhouette 3 each, which essentially means 5 instances of 3 vehicle-scale damage pre-armour for a grand total of 15 possible vehicle damage. The body I would consider silhouette 4. I also believe the target of the Throw portion of the Move Power also deals damage back to the object that is thrown, so if two Stormtroopers slammed into one another - they might deal 10 damage (presoak) to each other, and then another instance of 10 damage (presoak) to each other again, representing the mutual impact (not sure how many people subscribe to this belief though).

The only flaw with this is that an AT-AT has 40 H. Threshold, and 5 Armour. So a GM might have to rule bonus damage from another source (possibly Discipline check itself to add extra 'umph' to impact e.g. extra damage) and possibly mitigate or even negate the effect of armour - by logic that you are taking the thing apart, and not trying to blow it up with a rocket.

Secondly, a GM could rule that you can just pluck the limbs off of the AT-AT with a Discipline check as long as you had the 'rip things out of secure holdings' control upgrade for Move and enough points in strength. More of a narrative, than mechanical result - and probably the cleanest.

I am definitely sure Darth Vader has enough ranks in Willpower and Discipline, as well as pretty much the entire Move tree to easily facilitate a scene like this.

Yeah, move was my first thought too, but as you said, 5 armor, that's 50 personal scale damage and no crit rating, so I deleted my page of ideas and replaced it with a liberal use of the bind mastery for vehicle criticals.

Easiest answer would be "plot." It's Darth Vader, Dark Lord of the Sith, who is far more powerful a Force user than the PCs are ever likely going to be.

If going for a strictly mechanics-based approach, he used Move with enough Strength upgrades to affect something the size of an AT-AT (believe it's Silhouette 4), the Control Upgrade to remove secured items to rip the legs off, and then Magnitude Upgrades to affect multiple legs at once. The GM at that point simply narrated it that instead of dealing Hull Trauma to the walker, Vader's actions instead rendered the walker immobile due to lack of legs, thus putting it at Speed 0. He'd need at least 3 Force points (basic power, 4 Strength Upgrades, 3 Magnitude Upgrades), but it'd be doable, and given it's Vader, who's probably rolling at least 4 Force dice and is using dark side pips, it wouldn't be that hard for him to get the necessary Force points, even if he hadn't bought sufficient Magnitude upgrades. I don't know the XP costs since I'm away from my book, but it's going to be quite an investment in Move to pull it off. I as the GM would probably also require a Discipline check, with the difficulty set at the walker's Silhouette to literally "rip it apart." Now for Vader, said check would be quite easy (figure Willpower 4 and Discipline 4 at least), but a PC might find it more difficult unless they've similarly invested in Discipline.