Sign In Another Player
The "Halo 3" campaign supports local co-op playing. Sign in another account on your Xbox before you start the mission even if you are playing alone. During the mission, the game will automatically move the second player to your most recent checkpoint. If you die, keep playing as the second player until "you" respawn in a safe location near the second player. Playing in co-op mode helps you avoid losing ground when you die before reaching a new checkpoint.
Assassinate or Run and Gun
The first two rooms in the Cortana mission contain only a few Flood. Since shooting only attracts more Flood, use melee attacks to silently assassinate any Flood not looking at you. When Flood are present in corridors and small rooms, avoid losing energy or dying by running past them while shooting when necessary.
Stay Close to Doors
There are large rooms with swarms of Flood that are impossible to run through. When dealing with groups of Flood, stay near the room's door. The Flood will come toward you, but if you take too much damage, you have the option of hiding behind the door until your shields recharge. If the number of Flood coming for you dwindles, step through the door and fire to attract the attention of more Flood. After clearing the majority of the flood on the ground, pick up a long-range weapon to kill the creatures shooting down from the ceiling before entering the room.
Look Down
During the mission, you will go through rooms and tunnels with grates or holes serving as entrances to corridors. When this happens, take advantage of the Flood's inability to attack you by firing down on them. If you are able to attract a large number of Flood beneath a hole entrance, throw down a grenade to speed room clearing.