Halo (Halo 3)
The strats for this level are very similar for both easy and legendary. The beginning of the level focuses mainly on getting to the top of the centre tower as fast as possible. This is most easily accomplished by grabbing the gravity hammer from the closest flood when you enter the col, then grenade+hammer jumping off one of the pedestals up onto the tower centre support beam closest to where it goes into the ground. As you walk up the tower you can shoot a rocket or two over to your right at the door in the cliff where johnson is coming out. This kills johnson and skips some dialogue later. Walking up to the top of the tower will begin the fight.
The spawns for the tower fight are manipulable in that the location and speed of the spawns are related to the player position and the player's field of view. In order to maximize the spawn rate for the flood for both waves, make sure you are constantly moving and never looking in one place for too long. There are two auto-turrets by the door. You can activate them to help you deal with the flood, but make sure to save one for later in order to speed up the guilty spark fight slightly. On Easy, there is an invincibility underneath the centre support beam on the edge of the tower farthest from the door.
Once the fight is done, if you didn't manage to kill johnson before, he will jump down off the cliffs and go to the door and say some things. If you can headshot him before he gets to the doors you can skip his dialogue. Head on in to the guilty spark fight.
343 Guilty Spark FIght
During this fight you have two options: Wait for johnson to fire a laser shot at Guilty-spark, or perform the auto-turret jump to save about 10 seconds. Jumping off of autoturrets is slightly different than jumping off other equipment in the game, and many people get frustrated with this trick and decide not to do it. Regardless, trade your gravity hammer with johnson to get his spartan laser. This 'reloads' your gravity hammer. Once you have shot 3 laser shots at guilty-spark, equipment run your gravity hammer back towards the door at the exit of the area. Drop your gravity hammer so that it is leaning against the door. If done properly, when you load the next section of the mission you will be able to grab your gravity hammer through the door. This trick is very easy on MCC, and significantly harder on OG.
The Way the World Ends
Make your way back out to the tower fight area and up the cliffs on the right. You can perform some gravity-hammer slide jumps down the snowy slope if you managed to get the gravity hammer through the door after the spark fight, but just be careful not to fall off the edge. Every runner has fallen off the edge trying these slide jumps at least once. Make your way through the cliffs and try not to die. Once you get to the warthog you're ready for the fun part of the level.
The Driving Strats
The driving physics are different on OG and MCC. Most of the really difficult driving strats are easier on OG, but can be completed on MCC as well. The regular driving route is easy to do on both versions and is too difficult to explain well in text. Suffice it to say that the most important thing is maintaining top speed with the warthog, so don't take turns too sharply or bounce around or flip over. And exemplary driving route for Easy is shown in Prokiller's 9:47 run. On legendary, there are more panels that blow up so the route should be adjusted accordingly to account for the extra holes.