I made this Fiendish Leather farming guide to help out players who want to farm the leathers instead of buying them from the Auction House. I decided to search for places with high density of mobs which I can kill and drop [Fiendish Leather]. I traveled around Argus, and after a while I made a list of places.
Keep in mind! In Legion, several people can tag the same mob and still get loot. So, if someone tagged your mob but he didn't loot the corpse, you can't skin the mob.
Don't forget to enchant your gloves with [Enchant Gloves - Legion Skinning]! This enchant brings skinning time down from 1.5 seconds to 0.5 seconds.
Farming Duskcloak Pantharas is the best way to farm Fiendish Leather. You can stay on the spot marked on the map, and then you can basically stand in one place with a ranged class and just keep pulling mobs from left and right, they will keep respawning. These mobs cast a defensive spell which last for around 4 seconds and reduce their damage taken by 50%. With good enough gear that dmg reduction buff on the mobs won't matter that much, but if you have burst AoE abilites, try to always wait a bit if you see the debuff on them, it's very noticable because they become a bit transparent.
Exact spot coordinates: 73.5, 70.6
Farming Manafeeder Pantharas at Mac'Aree is a decent place, but this is a questing areas for one of the main story quests, so the place can be a bit crowded sometimes.
Defiled Stalker at Antoran Wastes is a great alternative if you can't farm at any of the mentioned places. There are only a few of them at the marked area, and they are a bit spread apart, but there is always at least 3 of them up so you can farm here without downtime.