Featured with permission by Kuhda
In this guide I will be teaching you how to farm Legendary cores in the most efficient way, and it can be done prior to completing the main story. The guide will be divided into two parts, first, opening up the area in which the unique monster that we will farm will be, and secondly, getting one Overclock Bangle. I do suggest being at a minimum of lv 55-60 as we will be fighting a unique monster.
Do keep in mind that for the second part(Overclock Bangle) you will need 4 skill fields, which are: Dark mastery Lv 4, Ancient Wisdom Lv 4, Ice Mastery Lv 5 and Focus Lv 5. However, these skills can be easily obtained, in fact, you get certain blades in the story that have these skills, you just need to level them.
For the first part: In Uraya belly there is a Unique monster lv 65 that will most likely drop a legendary core every time you complete a Driver combo(Break>Topple>Launch>Smash).
At first, this area will be blocked by a wall. In order for you to unlock it, you need to do a side quest called “The Titan Scholar”. This side quest can be obtained from a Nopon on Fonsa Myma’s port area. Once you accept the quest go ahead and Fast Travel to “Eight-Rock Skip”(Located in Uraya’s Stomach). There, look at the bottom of the little lake and you should see a cave(do not drop to the bottom, cave should be near cliff to the left side), go inside the cave and a cut scene will be triggered. After you fight off the monsters, the teacher of the Nopon will talk to you and say that he has gone to the belly before, but for some reason is blocked now, and he assigns you to the task to go see what is happening.
After the cut scene is over, you can go to the end of the cave, climb up, and you will see that you are on the Tree that is on top of the Garfont village. Close to the left side of tree there is a ladder, interact with it and Rex will set it up, this way you have access to the upper area of the Garfont village. Going back to the quest.
At this point, you have to just follow the quest mark. You can go to “Eight Rock Skip” again, fall to the bottom and go North. Once you reach the belly, you will see that is closed due to some enemies appearing in it. Defeat the enemies, and you will have access to the inside of the belly, which is where the Unique monster is.
You can farm him to get the legendary cores, however, we want to do as many Driver combos as possible in one fight, this way, we can at least get a couple of legendary cores per fight. This is where Overclock Bangle(Blade switch cool down) comes in handy.
One early Overclock Bangle that we can get can be found in Leftheria. However, in order for you to access it, you will need the Field Skills mentioned before(Dark mastery Lv 4, Ancient Wisdom Lv 4, Ice Mastery Lv 5 and Focus Lv 5.). This Overclock Bangle can be found in a chest on the “Isle of Sleeping Remains”. This area is pretty simple to access, you can fast travel to “Rigitte Harbor” and then after getting to the very first island just go right(to the island that is right on top of Rigitte Habor). There, in one of the big shells you will find a path downwards, there will be a black miasma in which you need Dark Mastery Lv 4 and Ancient Wisdom Lv 4 to disperse, after you do that, down the hall, there will be a little water cataract that you will have to freeze in order to access the Chest that contains Overclock Bangle, to freeze it you need Ice mastery Lv 5 and Focus Lv 5.
The Field skills can be found on the following blades obtained through the story: Ancient Wisdom can be found on Dromarch and Aegaeon, they both can get lv 2 Wisdom with ease, so it shouldn’t be a problem. Focus is found on Pyra/Mythra and Wulfric, at this point you can either have Pyra/Mythra have focus lv 3 and Wulfric lv 2, or just simply have an extra blade with Focus Lv1
Here is a video of a japanese player farming the monster: https://www.youtube.com/watch?v=UVpfEPjaKM4
Edit: Make sure you guys are using Gold chips(Increase the item drop rate by 50%) so you reliably get legendary cores, ideally, you want Gold chips in all your blades when doing this. I also have a common blade on one of my party members with treasure sensor. You can Check this guide from @studioleaks if you don’ know how to farm them. https://www.reddit.com/r/Xenoblade_Chronicles/comments/7i6fie/guide_how_to_farm_gold_chips_by_the_end_of/
Special thanks to BigSnoke for finding the video and Sligied13 for the Overclock Bangle location.