trick works in single or co-op. Go to the end of the level where you
meet a group of marines. Immediately run past them and go
outside. Now, hug the left hillside/wall and run in a clockwise
circle around the perimeter of this first part of the outside
you are running you will see a rock ahead of you and an area where both
sides of the hillside get close to each other... going this way leads to the
watery area where 343 Guilty Spark is floating around. DON'T GO
HERE. Cross over to the other hillside before you reach the rock, and
continue hugging the wall in a clockwise direction... you should be
headed back to the facility.
first video below shows where to go as you approach the facility after
completing this circle. There should be a bunch of marines just
standing around, not doing anything. They will no longer be trying to
run to the tower at the end of the level.
second video shows an odd marine position that can be seen when a flood
spore attacks a marine, and his friends prepare to open fire. (This can be
seen in pete_the_duck's 343 GS exploration here
third video just shows the general chaos that ensues when you roundup the
marines and lure some flood spores into the area.