This can be completed solo with a spare controller using a guest account
(2 controllers 1 xbox using guest) Video Below
Like the Guides above go to MULTIPLAYER, CUSTOM GAME, DEATH ISLAND
Set the Vehicle Respawn time to NEVER so your vehicles do not disappear
Now simple park the Tank on top of the Red Base an make sure the back tracks of the tank are on the raised area.. Now park a Ghost in front of the Tank.. You can also put the Ghost half on the Tank so it makes more of a ramp but it doe not always stay there..
Put your GUEST ACC below an Drive up the Ghost/Tank in the Warthog so you pop up into the air..
Then switch controller to your GUEST ACC an stand under the Warthog..
I got this in less then 15mins an just after the Game Finished!!!
Here is the Video of my successful attempt right when the game finished!!!
Different link added: -