This is the last level of Halo 3 "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. Don't kill the Pure forms in Crawler form climbing the tower, because they are invulnerable and will just climb back where they left off.
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. When you’re at the top of the building, pick up an equipment item called the Auto-Turret. These are great items, and Bungie is thinking of putting them in multiplayer. Press 
to drop the item, which is basically a robot that kills enemies.
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). 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 until Johnson fires his Spartan Laser at him, because you swapped your weapons for pathetic stuff the Flood used, and Guilty Spark can only be damaged by Johnson's Spartan Laser.
Go get the Spartan Laser from Johnson and shoot it at Guilty Spark 3 or 4 times. It doesn't matter if you miss because the Spartan Laser has infinite ammo. After he explodes, run out of the building and you'll notice your weapons have disappeared. Go to the place you left your thing and you'll see that they're still there. Pick up your gear and turn right of the entrance and go up the snow path. The robots have turned on you now, so kill them too. That's why you killed the auto-turrets; they would have attacked you also, and knowing their strength well, that wouldn't be a good thing. Go to the door at the end of the path, then run through and kill all Flood and robots, then you will find Johnson's Warthog.
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 robots trying to kill you. 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 last scene plays, and if you have finished on normal or harder, there will be another important video after the credits.