Shield drainer evasion
From Halopedia, the Halo wiki
Shield drainer evasion is a glitch that can be performed in the Halo 2 level, The Armory. It allows the player to stay outside of it while it drains your shields.
Walkthrough
Start the Armory on any difficulty. Go through the orientation process until you reach the recharging energy shields station. Once the "Hold A to start energy shield test" appears, navigate around to where the Master Gunnery Sergeant is standing. When the message appears again, activate it. Strangely, your shields will still go down even though you are not in the energy draining station.