Enemies in Space Glitch
From Halopedia, the Halo wiki
After the first spacewalk in the level, when you get to the ramp elevator, go to the stairs to the right and jump down past the Drones to the bottom. After you and your allies shoot a few of the Drones, the airlock will start to open and, if you are quick, you can rush into before any Elites or Grunts come out.
The doors will open and you can continue forward, but the Grunts and Elites will still shoot at you through the airlock, even though the Elites have no breathing apparatus. The Covenant forces will not leave it unless you push them out (which you can do to the Grunts if you have patience, but not to the Elites). Once outside the airlock, they will move around a little.
An easier way to practice this is to proceed normally and kill all the Covenant forces except for one Grunt. It helps if the Grunt panics and you start the elevator right away so the Marines do not kill the Grunt. Push the Grunt into the airlock. He will not shoot you inside the airlock, but he will once he is out in space. This "pushing into space" trick may work with Marines.