Shield drainer evasion
From Halopedia, the Halo wiki
On the level the Armory where there is the sheild drainer there is a glitch that allows you to stay out side of it while it drains your sheilds.
Walkthrough
Start the level on any difficulty. Now go through the part where it asks you if you want inverted aiming and head over to the recharging energy sheilds station. Once the "Hold E to start energy sheild test" appears navigate around to where the marine is standing and when the message appears again activate it. Strangely your sheilds will still go down even though you are not in it.