What is a quick way to farm demon skin on lvl 70? [duplicate]

Legendary materials are usually found as drops off super-unique monsters (the ones with the purple names). Demon Skins can be found on the following monsters:

• Axgore the Cleaver (Phase Beast): Act III, The Core of Arreat
• Haxxor (Phase Beast): Act III, Tower of the Damned Level 1
• Khatun (Sky Terror): Act IV, Gardens of Hope Tier 1
• Razorclaw (Sky Terror): Act IV, Gardens of Hope Tier 2 (spawns during the quest, The Light of Hope).

Because they drop off super-unique monsters, you won't be guaranteed a chance at the drop when you create a new game as the monster may not even spawn! Be sure to pay attention to bounties that target these monsters, as the bounty guarantees that the monster will spawn.