Achievement is really difficult. So I recommend finding the enemy ai or players last units then use a leader power to destroy them. Professor anders leader power not recommend for this achievement. Note: dosent have to be the last surviving units in game.
Achievement is difficult but do able. You have to complete the optional objectives on the mission. Then a green circle will flash over an area on the map. Send units ( particularly air units) to collect the skull. Note: any units can pick up the skull but there are skulls that cannot be collected by ground units.
The acheivement tells you to just beat campaign on Legendary. Its best to go through and collect all the skulls on a lower difficulty then go back through on Legendary with the skulls on that power up your allies and weaken enemies. Since the acheivement says to just beat the game, you can use skulls.