I made this Cragscale farming guide to help out players who want to farm the leathers instead of buying them from the Auction House. I traveled around Nazjatar to search for places with high density of mobs which I can kill and drop Cragscale. Below you will find a list of places I think are the best for farming Cragscale.
The rank 1 requires Skinning 140. You can learn it from your trainer Rolm / Instructor Ulooaka at Nazjatar. (You have to finish the first few intro quest at Nazjatar before you can see these NPCs.) You can still skin the mobs without getting the rank 1, but you will get fewer leathers.
You can get the rank 2 from Vor'koth. To summon him, you have to loot a Chum from beasts in Nazjatar. (random drop) Then toss the Chum into the Eel Infested Water at this location. (Coordinates: 48.35 24.31) Kill the 3 mobs that spawn, loot the chum then throw them in again. Repeat this several times then Vor'koth will spaw. It's a rare mob so you might not be able to solo him if you have bad gear.
Both the Rank 2 - Skinning Technique: Dredged Leather and the Rank 2 - Skinning Technique: Cragscale are dropped by Vor'koth, so I had to kill him twice to get both recipes because I could only loot one of them from one kill.
Rank 3 is from a quest item that drops from Skinning Cragscale. (random drop, not 100%)
Nazjatar - 1
The best place to farm Cragscale is at the Forgotten Tunnel in Nazjatar. The Deeptide Frenzy in the small lake has a fast respawn rate.
Nazjatar - 2
Kill Abyssal Eels, Coral Slitherers, Deeptide Frenzy and Razorshell Turtle around the marked areas.
While I was farming there, the respawn rate was fast enough that I could just farm at the area marked with number one, but if you are killing the mobs too fast you have to move between the two areas. (kill turtles along the way between the two).
The small Abyssal Spawns don't drop any leather.
Nazjatar - 3
Kill all kinds of beasts around the marked area. You can't skin the baby turtles and the crabs, so skip them if possible.
Nazjatar - 4
I only recommend this to ranged classes.
Kill Coral Skate, Fathom Ray and Fathom Ray Broodkeeper around the marked area. They are mostly up in the air, so you have to look up to pull them.