Weaponless Elite glitch
From Halopedia, the Halo wiki
The Weaponless Elite Glitch is a glitch which can be done on the Halo Combat Evolved level Assault on the Control Room, in Halo 2 levels Metropolis and Gravemind and Halo 3 level The Covenant.
Walkthrough
Assault on the Controll Room
Go through the level until you get out of the caverns, walk up the big slope and reach another canyon with a giant pillar in the middle. Walk inside it and you should see a Stealth Sangheili. When he notices you, run out and he will follow you. Sometimes the Elite will run straight outside if there is a ghost there (if the player brings one from the first part of the level, the one parked outside by default will not attract him). The Elite will get into the ghost and will try to shoot and ram the player. If the player manages to tip over the ghost, the Elite will fall out and will have no weapon and will not even move or react to the player, unless he gets close, where the Elite will try to melee with his bare hands as if he had an Energy Sword.
Metropolis
Go to the level Metropolis in Halo 2 and board the Scarab. Throw a plasma grenade at the Scarab operator Elite while he is operating the Scarab. He will execute the animation for drawing an Energy Sword but will not actually possess one. Easy is the best difficulty for this.
Gravemind
Get the Catch and Sputnik skulls and go to the level Gravemind. Make sure that the difficulty is on Easy. Go to the first outdoor area past the Hunters where the Honor Guard Elites and the Brutes are fighting. You should take out the sniper Jackals before attempting to perform this glitch. Get the SpecOps Elite toward the right to throw a plasma grenade at you. Either shoot a Brute Shot at the Elite's feet or jump above his head, but just enough so he does not stick you, and cause him to throw the plasma grenade up in the air. If done correctly, the plasma grenade will land on his head and not you, and he will execute the animation for pulling out an Energy Sword, and again will not actually possess one as this Elite is Special Operations and is not scripted to have a secondary weapon. Instead, he will pull out an invisible Energy Sword and will follow you around the Valley of Tears. When you reach the part where the Flood-controlled In Amber Clad comes, he will not follow you across the light bridge but stand there staring at you.
The Covenant
When the Separatist Phantom arrives to pick up the Arbiter, use an Energy Sword to melee and destroy the Plasma Cannon platform. The Elite operator will not take any damage and when he gets off the turret, he will have no weapon. If you stick him with a plasma grenade, he will take out an Energy Sword and usually die. If the Thunderstorm and Mythic skulls are turned on and you stick him with a plasma grenade, it is very likely that he will survive. If you betray your allies, he will attempt to melee you with his fist and his melee does damage.
Explanation
Because the Elite's grenades are weaker than yours, it will not kill him but will make him execute the animation for drawing an Energy Sword, causing him to become "weaponless".
