Save Marines on Sacred Icon
From Halopedia, the Halo wiki
In order to save the Marines on Sacred Icon, you will need to acquire the Sputnik Skull. Play the level until you get to the part where you hear the Marines saying something along the lines of, Marine Sergeant, "Proceed to the objective, we'll hold out as long as we can." Marine Private., "Uhh Ahh! Get it off me!", "Suppressive fire, Suppressive fire!" At this point, Explosive Jump across the gap to where the Marines are. This is very difficult to pull off, as a single hit from any Flood forms will kill the Marines. However, if some manage to survive, you will notice that, unlike in the level The Great Journey, the Marines are your enemies. This is the only way to fight Marines without betrayals or mods as the Arbiter in Halo 2. It is recommended to take the Sentinel Beam, because it has a range advantage, allowing you to take out the Flood rather easily.