Fall damage reduction
From Halopedia, the Halo wiki
Steep Landing Method
In any Halo: Combat Evolved Campaign level, it is possible to survive fall damage when jumping from great heights. A player must simply jump from a high ledge or cliff and land on a steep, slanted surface—one that they would not be able to stand on normally. There is no friction on such surfaces, which is why players can survive such landings. The trick works best when landing on a steep, slanted rock, or on a Wraith. However, the trick has its limits; when performed from too high, the player will automatically die, and it will not work.
In the Halo: Combat Evolved level Two Betrayals, the trick can be used as a shortcut; when going to retrieve the Banshee at the beginning of the level, players can slide down the slanted portions of the walls of the control room and effectively skip the walk down.
Vehicle Entry Method
In the Halo: Combat Evolved level Keyes, there is a part where the player is on the third floor of a structure, and must retrieve and enter a Banshee. The player can jump down to the second floor, kill the Elite manning the Shade, and jump down onto the Banshee. If the player enters the Banshee right as they land on top of it, they can survive some fall damage that would otherwise kill them, while at the same time avoiding a large number of Flood and Covenant forces.
Corner riding is a Halo 2-specific technique developed by the Halo glitching community. To corner ride, a player must repeatedly move back and forth over a 90-degree vertical angle. This periodically "tricks" the game into believing the fall timer has been reset, preventing death.
Probably the trickiest way to avoid fall damage is crouching. If a player manages to crouch just before impact with the ground, their shields will take little or no damage at all. Such a technique requires large amounts of practice, however, as the timing is very tricky and failure to time the action properly can result in death. Imperfect timing can still lead to survival, albeit with lowered or drained shields; however, horrible timing can in some cases increase the fall damage that the player sustains. Ironically, this method is actually very similar to what the Master Chief does in the cutscene after he kills the Prophet of Regret.