I think you have to decide how tough this should be first.
From the GHWiki,
Nerull is the Flan god of Death, Darkness, Murder, and the Underworld, worshiped on countless worlds. He is known as the Reaper, the Foe of All Good, the Hater of Life, and the Bringer of Darkness. His symbol is a skull and scythe.
Nerull - Ghwiki
I think you have to settle in your mind what they are achieving. A simple strait-forward answer might be:
"Simply supplant Nerull in the hearts and minds of all his worshipers on the countless worlds."
Ask yourself how Nerull got to be a deity, against what odds, and after how long. What could the player be or do to have an easier time him\herself?
Does the player want your help to simply walk in and take his desk? Does your player want you to carve a zipper through your world design so that a player can become a major god in a couple of dungeons? It's not just your problem to figure out. If your deities simply change in a month and a half of adventuring what should your world look like?
You and your player should decide what the players strengths and abilities are. Tell the player the universe is not labeled with tabs that say 'how to become a deity and depose Nerull'. What can you decide together that might be reasonable goals - IS THIS REASONABLE?
How long will you both play a Roleplaying game if you need this level of achievement to have fun? What will you guys get together to do next?
I'd say you need to find a structure for godhead and start there.
What sort of character does he have now?
S