From Halopedia, the Halo wiki
A team of four players has 20 minutes to complete a full set, along with a bonus round at the end of the set. Players have unlimited lives and ammo, and any weapon they pick up (be it from a fallen comrade or enemy) will also have unlimited ammo.
On the first and last wave of each round, a Phantom will drop off that wave's enemies. The Phantoms also bring along with them an enemy vehicle, however which vehicle they bring depends on the map being played. if players reach the bonus round they will face drop pods and a phantom that drops only grunts until the ninety seconds runs out.
It is highly recommended to use the jetpack loadout, that way the player can get around the map easier and utilize heavy weapons to greater effect.