Phoenix - Borderlands 3 Farming Guides
Location: The Splinterlands on Pandora
Part of: Crew Challenge
Drops: Phoenix Tear (Artifact), Deathless (Artifact) and Malak's Bane (Dahl Sniper)
Phoenix is a Target of Opportunity Crew Challenge which you can find in The Splinterlands on Pandora. The closest fast travel location to this is The Splinterlands - Pit Stop; hop in the Catch-A-Ride and drive down to Phoenix. You may be able to guess based on the name of this enemy that it will reincarnate a few times throughout the battle. Best advice I can give is make sure it dies the final time (3rd death) in a spot where you can easily get the loot.
As far as loot goes, Phoenix Tear (Artifact), Deathless (Artifact) and Malak's Bane (Dahl Sniper) are all on his dedicated drop loot list. The Phoenix Tear is an item more unique to the Phoenix and difficult to find elsewhere.