Doc Mercy is Named Enemy in Borderlands 2. He is located in the Three Horns Valley region on the Planet Pandora. You will encounter him inside the Shock Fossil Cavern area during and after the side quest “Medical Mystery” or during the Digistruct Peak. You can use a geyser outside the Happy Pig Motel to quickly launch yourself inside the Shock Fossil Cavern.
You can farm Doc Mercy for the Infinity as he is the dedicated loot source for this item. Doc Mercy’s legendary item has a 10% drop chance.