In addition to the type of achievements that are standard across all Halo titles, we took the opportunity to create several level-specific achievements. This specific batch was born out of community meta experiences spanning the decade since Halo: CE launched in 2001
I think they're going easy on us.
12 minutes is GOOD, even if you did grenade jump most of the doors, trick trough some areas and have a handful of near-to-death-situations.
Just think about the amount of rage this achievement might cause among the "Full GS!!!!" players!