You don't need grenades or any other equipment to jump for the skull, all you need is a 2 scorpion tanks.

The scorpion can climb through the rocks for going up.

Use the 1 scorpion for the other scorpion, so it can cross and go to the road above (make sure that the one turret of the scorpion is hanged on the road).

When you are on the road now, just locate the skull and use the tank as a base so you can jump for the skull.

You need to kill all your allies before doing this, because they can get the scorpion and it will ruin your plan.

Anyway, you don't need them anymore when you got the skull!