Okay so after deafting the covenant in the first part of the game - a marine ship flies down and drops off 3 tanks for you and your men, right? Okay. Then you drive those tanks to the 2nd section in the game where there is a structure with 4 support beams. Normally you drive through the canyon and UNDER that structure. Go get up on that structure and go down to the last support beam - the beam has an opening in it that kind of makes a triangle shape. Inside that open triangle area at the very end, is the skull.
It took me FOREVER to get the grenade jump right - and that is the only way to get to it.
I tried forever on the left hand side of the beam with the grenade jump and couldn't get up there. But when I tried the grenade jump on the right hand side of the beam - Bam! I got up there.
Believe me, you will get ****ed at trying to achieve getting up there.
I hope that helps.