McNally is Named Enemy in Borderlands 2. He is located in The Dust region on the Planet Pandora. You encounter him during and after the side quest “The Bane” which becomes available after you complete the main story mission “The Once and Future Slab”.
You can farm McNally for the Hammer Buster as he is the dedicated loot source for this item. The Black Queen’s legendary item has a 10% drop chance.