This is the last level of Halo 3, called "Halo". You start out crash landing the Pelican in the snow. You walk for a bit until you get to a big open area covered with snow. There's a building in the middle. You keep walking but then Flood capsules fall from the sky. Kill the Brute Flood with the Gravity Hammer and use melee weapons to destroy every single Flood that falls from the sky. Grab a Brute Shot if you find one and use that instead, because the Hammer partially blocks yours view. The Fuel Rod Gun is one of the best weapons for this section because of it is as strong as the Rocket Launcher, with even more ammo. Don't bother trying to kill the Pure forms climbing the tower, because they aren't attacking you, and they will soon respawn, anyway.
Now head around to the right of the map, killing more Flood. Johnson will finally show up and help you kill some Flood with his trusty Spartan Laser. He's not every effective, and take care that he doesn't accidentally shoot you. At the top of the building, you will find a new piece of equipment called the Auto-Turret. Drop them with and they will float in place and attack the Flood with a powerful sentinel beam.
It will help you kill Flood, but if it gets stuck to the floor or just floats there and does nothing, walk into it and it will start moving. There are two up there, so use them both. Sometimes they will do all the work for you, however, if you're trying to get 15,000 points don't use them. Help them kill the waves of Flood that come. The Pure forms that were crawling up the twoer now come in Tank form to destroy you. Don't panic; they eat all your Shotgun shells like they're nothing, so melee them with the Brute Shot.
When they are all gone and Guilty Spark opens the door, don't go inside yet. Destroy the Auto-Turrets you dropped. Also, swap your Brute Shot and Gravity Hammer with something the flood dropped. Remember where you leave the items, ideally on the top floor, as you will be able to pick them back up later. Go inside and run to the last door in the building. After a speech, Guilty Spark turns on you and you have to kill him. Avoid Guilty Spark and his laser, and eventually Johnson will shoot him with his Spartan laser. This will knock Spark across the room, allowing you to take the Laser from Jhnson
Once you have the laser, continue avoiding Guilty Spark's lasers, while hitting him with your own. It will take 3 or 4 shots from the Spartan laser to take him out. It doesn't matter if you miss, because the Spartan Laser has infinite battery power. After Spark explodes, walk over to Johnson to trigger a cutscene.
The Way the World Ends
As you leave the control room, whatever weapons you had before will be replaced with an Assault Rifle. You will fight a few Flood combat forms on the way out, and you can take their Plasma Rifles if you want. On Higher difficulties, these are useful for quickly eliminating Sentinels. You should now go to the place where you left your weapons before fighting Guilty Spark. You'll see that they're still there, just as you left them (hopefully). Pick up your gear and turn right of the entrance to the control room. There is now a snowy path leading up to the cliffs. You will have to fight more combat forms, plus sentinels, ans you head for the door further down the cliff. A good strategy is to let the Sentinels eliminate as much of the Flood as possible, and take out whoever wins.
Once you reach the door, you have a room of Flood and Sentinels to fight. You'll then be faced with a long hallway full of carrier forms. You can take them out from afar, and then deal with a swarm of infection forms, or just try to run and jump over them. Fight through them, and once outside you'll find Johnson's Warthog. In single player, there will be one Warthog, but you will find two in co-op.
If you're playing co-op, the best driver should drive or if the Warthog tumbles, leave one guy behind to save time as behind you tiles explode or collapse. In co-op, multiple Warthogs appear throughout the respawn points to give anyone left behind a chance to escape. Now you must drive out of the level before it explodes. Look out, because the floor falls and there are flood and sentinels trying to kill you. Avoid running into carrier forms, as they will flip your Warthog when they explode. To get to the frigate, floor the accelerator on the Warthog to gain enough height to jump from a giant ramp and into the hangar. The final cutscene plays, and if you have finished on Normal or Heroic, there will be another important scene after the credits. On Legendary, you get to see another short scene after this.