Floating assault rifles
From Halopedia, the Halo wiki
On a level with a Pelican, such as 343 Guilty Spark for example, it is possible to stay inside the Pelican. To see this trick, throw grenades before being dropped off by Foehammer to stay on the Pelican on 343 Guilty Spark.
Procedure
After you get off the Pelican, quickly run to the area in which you go into the structure with the Flood. When playing the level normally, you would eventually see several Covenant running from Assault Rifle fire coming from inside a structure. Go around the level area and you will eventually find yourself above this structure. As the firing begins, jump down and enter the structure and you will find that no one is holding the assault rifles. The explosions are not caused by frag grenades either, rather they are spawned effects. This saves a lot of time instead of going through the entire swamp.
Alternative
An easier way to get to the floating assault rifles, is to go to the front of the pelican, and there should be a big rock. Jump on the rock, and from there jump onto the nose of the pelican. (Do this quickly before the pelican flies away.) Now try to stay in the middle of the top of the pelican because as it flies away, your character will stay in the same spot. Keep moving around the pelican so you don't fly off. Wait until the back wing is close enough to the cliff nearest to the pelican, (this will take a few tries so just keep respawning back to your last checkpoint if you fail.) and crouch jump onto the cliff. If timed correctly, you should NOT slide down the cliff.
