I got this from a FAQ on gamefaqs.
Level: The Storm
In the first open field area where you must kill the Wraiths, don't
kill the Wraiths. Kill the first three Ghosts, then the turret gunner on the
AA Wraith, then wait for two more Ghosts to drop in, and kill them. That clears
the field of almost everything but the Wraiths. Contrary to what I've heard,
you can actually kill the AA Wraith (other sources say you shouldn't kill
either), but all I know is that if you kill the normal Wraith on the platform,
that will cause the skull to disappear.
So as for getting the skull, it's tough when you have that normal Wraith
reigning artillery fire upon you. So the first thing you need to do is drive
up the ramp beside it and get on the platform with that Wraith. Kill the
gunner and then do follow the Wraith by sticking close to it so that it doesn't
blast you. Here's the key: stay close and make sure your back is facing the
field. What you want the Wraith to do is make a dash into you, causing you
some damage, and it should blast toward that ramp, but hopefully the Wraith
will get stuck in the gap between the ramp and platform. If done correctly,
the Wraith will be caught in place and unable to get out.
Now is skull time. Go grab a Ghost and take it to the round structure next
to where the AA Wraith was sitting. Notice that the shortest part from the top
of the structure to the ground is facing the platform, hence why we had to
get rid of the Wraith. Back this Ghost up so that the smooth front is facing
that general direction, and park it right next to the structure. Now go to
the ramp at the other end, flip over a downed Warthog, drive it back over to
the other side, and use the Ghost to prop the Hog up. Now jump on the Hog and
get up there to claim your prize.
The whole point of this is to get on top of that structure to claim the skull.
If there is an easier and more sound way of doing this, I have not yet heard
it.