Run Whispering Sepulcher or Abandoned house ad Infinium friend. They are the easiest portions, and drop roughly 7-10 Extract per run. They also can drop salts, and the Trash King boss drops crystals occasionally.
For me the only gear I wanted was the Dragoon Hat, Dragoon Boots and Dragoon ring. The easiest way in my opinion to farm the Dragoon gear is to start with Abandon house. This dungeon is great because it drops all the reagents necessary to craft the Dragoon Armor. It drops Extract, Salts and Crystals. Not to mention it drops the Vanguard Hat as well which makes it faster to craft the Dragoon Hat. After getting the hat dropped I would then farm Vigor's tomb. This area drops a ton of Salts and can drop crystals. This place drops the Vanguard boots as well which will make crafting the Dragoon Boots easier. You can do Vigor's tomb after you beat Foetid Crypt and the Whispering Sepulcher. A quick tip if to make this faster is when facing Sir Greaggor in Foetid Crypt I highly recommend using the Treasure Card Doom and Gloom while spamming Aoe hits such as Tempest or Bugs. After you have beaten this and farmed Vigor's Tomb for the boots I would recommend farming Whispering Sepulcher for the rest of the reagents. However if you have little to no crystals for any reason and not much Salts I would farm Abandon House even if you have the Vanguard Hat because the Reagents are great there.