To approach the final base it says you need to be in a certain vehicle but names it differently which can be confusing if your anything like me.
You have to rock up in the Xerxes III tank otherwise the doors wont open to begin the mission.
Just a tip, hope people find it useful. the rest of the level is fairly linear. Achievement will pop after the credits (you can skip them) and when you select to resume game from the main menu. Good Luck.