From Halopedia, the Halo wiki
The Marine Corps Gives Its Regards is an achievement in Halo: The Master Chief Collection. It is unlocked by beating the par scores on all Halo 3: ODST campaign levels. Mombasa Streets levels do not feature campaign scoring and as such do not have par scores. The achievement is worth 20 Gamerscore. Though it can theoretically be unlocked on any difficulty, more points are awarded in campaign scoring when playing on higher difficulties.