lynch

1.25 billion = 1250 million.

1250 million / 100 million = 12.5, so the average achievement value is ~ 12.5 gamerscore. Very believable.

On the final level of Halo 3 (named "Halo") you can opt not to drive the hogs an instead find four ghosts in a cave. If you complete the level, with iron skull on (you die, everyone goes back to checkpoint. you all die, level restarts) and everyone in the ghosts, you get the achievement.

Sounds pretty tough IMO.