In this level victory is determined by how many of the enemy you
kill. The harder the difficulty level, the more waves you face, the
more you have to kill.
So it is not necessary for you to protect your baggage at the
top of the hill at all. You can wander the map at will. (And get
butchered in the process.)
It is interesting to note that even if you evacuate the hill,
you can still see all the enemy units on it. Who is keeping an
eye on the enemy even though you aren't there?
Your Invisible Observers are!
In this level Bungie put this special unit type on your side.
These units are probably just under the mesh of the map so that
they don't show up as friendly dots on the display. Somewhat like
Thrall not showing up when they are in water.
Because the units are on your side, you see everything around
them. So in this map, when the forces of Dark storm the hill,
you see them no mater how far away you get.
And what do these units look like? Very tiny Warriors!