This CAN be unlocked using the BANDANNA SKULL. I know the game says achievemebts will be disabled, but that only appears to be the case for level, difficulty, score, and par time achievements not misc ones.
Just like in Halo CE Anniversary, grenade and shotgun everything, using the plasma pistol for shields and sentinels. I ran through the level pretty quickly as the shotgun and plasma pistol are really the best weapons for the level anyway.
Please note that the achievement did not pop for me immediately as this game was having its achievement problems so I thought it couldn't be unlocked with bandanna just to find today that it was a delayed unlock and unlocked this morning. Happy hunting!