High Charity Walkthrough
From Halopedia, the Halo wiki
Fighting begins immediately as the level starts - a group of hostile Flood Combat Forms and a few Jiralhanae will be fighting one another directly in your path. If you have not moved and instead opted to wait for them to fight, the victorious faction (almost always the Flood) will proceed to seek you out. Swap out your Plasma Pistol for another weapon when possible - it is not useful against any of the enemies encountered on this level. An Elite Combat Form will usually appear with a Type-1 Energy Sword, which is a good replacement. Alternatively, dual-wielded Plasma Rifles and Brute Plasma Rifles make for an effective combo.
Cut down any Flood forms if you get the sword, but avoid using it against Brutes, as it will waste your charge quickly. Once the area around the downed Pelican is clear, scavenge the wreckage for grenades and a Shotgun if you can find it. If you need heavier firepower, take a Rocket launcher. While there are no vehicles to shoot down, it might come in handy when getting rid of large groups of enemies. Your main enemies in this level are Combat Forms and Brutes. Drones, Grunts, and Jackals also appear.
Anyway, the best weapons to have is a sword and shotgun. Sword for Flood, and some shotgun to teach those Brutes a fatal lesson. If you wish, you can pick up a Beam Rifle for later long-range work. Once you're set, head through the only door off the platform, which leads to a sloping room. Four Brutes will appear from the other side, so lob a plasma grenade and hope you've stuck one of them as you duck back behind the small wall near the entrance. Lob another, if you've got one. Hopefully, two Brutes will remain, who will be engaged by Combat Flood. Retreat back to the platform to gain some breathing room if things get hot, then charge back in and take a shotgun to anything still moving.
Don't worry if Cortana says, "We don't have time for this, Chief", because there's no timer. You could probably loiter around for five hours and Truth's Forerunner ship still won't leave till you're aboard, unlike the Warthog Run at the end of Halo: Combat Evolved. Anyway, back to the battle. Head through the door at the opposite end of the sloping room and get on the lift and ascend. Move forwards and grab a beam rifle and head up the ramp. A Drone swarm will flutter past the windows of your location and head towards the platform on the left to engage three of four Combat Forms. Zoom to 10× and snipe all the Drones. Another swarm will appear. Snipe to your heart's content. When only the Combat Forms remain, head back into the corridor and recover whatever weapon you exchanged for the beam rifle. If your playing on Legendary, you will see more drones pass though the door to attack. Make sure to leave as soon as the last drone falls.
Afterward, walk up to the hologram on the platform and turn on the light bridge. Check if the other platform is mostly clear. Once you feel it's safe to go, cross the light bridge and activate the other light bridge and cross it. Once you arrive on the next platform move toward the door near the wall. But get your close-ranged weapon ready! Combat forms will be leaping out of there any moment. Before entering the hallway past the door, be sure to walk over the shotgun if you need a close-range weapon or have a shot gun on low ammo.
When you enter the hallway, you'll notice that Flood spores are obscuring your view. You can turn on your flashlight, but for the plan that is recommended, It's not useful. In the following passages, Flood will constantly spawn, so the best advice that can be given is to keep on moving. You will soon enter a passageway with a lower middle and two sides that are slightly higher then the rest. It's recommended to jump up top because Brutes usually walk across the middle part. Go through the hallway and you will reach another one-except they'll be a lot more Covenant in here. Once again, jump on the slightly rise path and continue to the other side of the door.
You'll enter a room where the air is clearer and that's packed with Covenant. It's recommended to just keep on the path and cross the room. Staying there too long will bet the Covenant's attention and will spawn the Flood. You will enter another room, almost the exact same as the other-but this room is filled with the Flood. Once again, keep to the path and only shoot if something is to close for comfort. You'll enter a winding hallway, but it'll be empty. Take a right and get on the elevator. Activate it and wait for the elevator to finish it's ascent.
Here, an epic battle between the Flood and the Covenant will take place. Stay back and wait for the battle to more or less finish, and then jump into battle. If you waited long enough, most of the combatants will already be dead and the survivors will be seriously injured - thus, this will be an easy battle. Once you are the only living creature there, Cortana will open the doors and you can proceed. When you enter the Room, make sure to near the glowing table thing-there's an energy sword there, but it's easy to miss. You can also take the beam rifle if you want, but do note that this is sacrificing medium range. Go on the elevator and get your fingers ready to battle.
Once you reach the top, you'll notice Flood and the Covenant fighting directly opposite to your position. If you're playing on Normal difficulty levels,then feel free to just run right across the platform to the other side, and you'll be done with the level. If you're on Legendary, then you have to be the last survivor in order for you to be able to finish, unless you are very lucky.