This is a walkthrough for the level Cortana, structured for play on Legendary difficulty with notes for lower difficultiy levels. Cortana is the penultimate level of Halo 3, and arguably one of the most difficult of the franchise. Your mission as stated is to find Cortana on the High Charity, free her from the Gravemind and eliminate the Flood. Using Skulls on this level is not a good idea for inexperienced players, but using the Catch skull provides you with a distinct advantage, as the Flood do not use grenades, but upon death, they will almost always provide them with this skull active.