Here is some information regarding the Narrative Navigator challenge, and how completing missions on different difficulties rewards different amounts of mission completions.
> Narrative Navigator:
> Complete Campaign or Spartan Ops missions. Earn additional progress for completions on higher difficulties.
I have completed the Halo 2 mission Uprising on each difficulty to test this.
- Easy - 1 mission completion - Normal - 2 mission completions - Heroic - 3 mission completions - Legendary - 4 mission completions?Unfortunately I was at 6/8 missions completed after my Heroic playthrough, but presumably beating a mission on Legendary would be worth 4 mission completions if the pattern stays the same.