Go onto to sandbox on FORGE.
Sell EVERYTHING on the map.
You don't need anyone else with you but its more fun if there is.
YOU MUST SELL ALL RESPAWN POINTS.
Make a trip mine and set the respawn time Don't forget to change all the numbers of mine you 
can to 1.
Then make a big pile of trip mines.
Be careful you don't stand in the pile Its easier with two people becasue you can build the 
pile quicker.
Then after you have made a big pile if all the grids have gone you've overloaded the map and 
the guardians of.
then turn to face the sun and then drive towards the pillar to the right of it and look about 
you should find the skull

Thanks for reading 
Never give up looking for new ways to stopping the covenant