THATS MY COOL MAN I CAN'T TAKE IT OFF!
Back Pocket - Kill 3 elites on Keyes with the sniper rifle after the banshee escorts arrive. On legendary.
Jimmied - Get by any locked door in the campaign.
Alright Then... - Kill every single flood entity in The Library, on legendary.
You Snooze, You Lose - Steal the far banshee on the snowy bridge on Assault on the Control Room, on legendary.
Landmines - On legendary, kill three elites with the same grenade explosion.
Invisible Is Nothing - Kill all the camo flood in the armory on The Maw legendary.
They're Still Faster - Complete Assault on the Control Room legendary without ever using a vehicle.
Pressure Point - Kill 30 elites with sniper headshots in a single run of Truth and Reconciliation legendary.
Don't Blink - From the time you enter the engine room (when the 'Light Fuse, Run Away card flashes), finish the remainder of The Maw in 10 minutes or less, on legendary.
I think it'd be cool if 'secret' achievements were included, like in Madden.