Outside of the Kurast Dock of the Dark World, Abel wiped out the Spider Forest, Great Marsh, Flayer Jungle, Lower Kurast, Kurast Blazaar, and Upper Kurast.
Of course, the hell creatures on the surface were the easiest to kill. With Johnson's Titan gear as well as Flying Flame and White Snow as support, all Abel needed to do was to wait for the souls of those hell creatures to come to him.
He found Khalim's eye in a tunnel in the Spider Forest and Khalim's brain in the Flayer's jungle. He entered the sewer of the Kurast Blazaar and found Khalim's heart on the second level.
Exploring those tunnels took a lot of time. There were many hell creatures, and he had to be careful. He wouldn't risk his life.
With the spirit guardian knights as a shield, he gathered the remains of Khalim. He then combined those remains with his Horadric Cube into Khalim's will. While destroying the Compelling Orb as part of the quest, the path of Mephisto opened up.