You can't. The last 3 skulls can ONLY be done on Legendary. Additionally, if you are asking for tips you probably aren't ready for Legendary. For Lost Platoon, chopper stealing and storing in the laser spawn is the #1 thing to do. Good luck even getting your first chopper.
Posted by: Insane Monx
I don't think it's a good idea to do it on Legendary Solo.
Try Normal or Easy.
Or get a group and then do it on Heroic.