The Bridge Descent / Level De-loadEdit
It is possible to drop from the first bridge in the level to the canyon floor below, which breaks enemy spawn triggers for the rest of the level. The trick has 3 parts, 2 of which have alternate easy and difficult methods.
The positioning and movement for each drop is quite precise and is best presented in a video tutorial:
The trick differs between NTSC/PC and PAL. Rockslider has found a variety of PAL-only methods for the bridge descent.
There is one place later in the level where a grenade jump can save 5 or more seconds. In the first room after exiting the first banshee, as you enter the inner part of the room, you can do a single grenade jump up to the platform on your left. An example video can be seen here: http://www.youtube.com/watch?v=SrtPhA80zIQ#t=4m42s